Key Issues in Printing and Dyeing Wastewater Treatment in China
Since the beginning of the 21st century, the country has paid more and more attention to ecological environmental protection, and has increasingly tightened the control of wastewater discharge standards and total regional wastewater discharge. In order to ensure the sustainable development of the textile printing and dyeing industry, the printing and dyeing wastewater treatment technology has also continued to show new changes, and Many new problems have emerged. The reasonable solution of new problems in the treatment of printing and dyeing wastewater is the only way for textile printing and dyeing enterprises to achieve a low-carbon economy and sustainable development in the future. Based on the analysis of the current status of printing and dyeing wastewater, the author explores the current treatment technology and corresponding problems of printing and dyeing wastewater, and proposes possible solutions and solutions based on existing research. Benefits.
1. Analysis of the status of printing and dyeing wastewater
Dyeing wastewater discharge
China is a big textile country, and the textile industry plays a huge role in our national economy and people's livelihood. At the same time, the textile industry is a heavily polluting industry, especially the discharge of printing and dyeing wastewater has jumped to the top 4 of the national industrial wastewater discharge. Water consumption is 100 ~ 200 t per 1 t of textile processing, of which 80% ~ 90% become waste water. Statistics show that in 2008, the textile industry wastewater discharge ranked third in various industrial industries, accounting for 10.6% of the total industrial wastewater discharge in the country, and the annual wastewater discharge was about 2.3 billion tons. Of the textile industry wastewater, printing, dyeing and wastewater are the major sources of pollution, pollution, and pollution, which account for 80% of total textile industry wastewater discharge. In addition, the production process of most small and medium-sized printing and dyeing enterprises in China is still at the level of the 1980s, and the utilization rate of water resources is far behind the rest of the world. In the case of producing similar units, the content of pollutants in printing and dyeing wastewater in China It is 2 to 3 times that of foreign countries, and the water consumption is as high as 3 to 4 times.
Dyeing wastewater quality
In recent years, with the scientific and technological progress and continuous innovation of the textile printing and dyeing industry, the use of chemical synthesis raw materials (dye and auxiliary) has greatly increased, which has caused a trend of diversified and complicated water quality of printing and dyeing wastewater. The discharged printing and dyeing wastewater contains a large amount of slurry, dyes, auxiliaries and surfactants, etc., resulting in high alkaline and chromaticity of the wastewater and low biochemical properties. At the same time, printing and dyeing wastewater is not limited to high COD, high chroma, and low biochemical characteristics. Some printing and dyeing wastewater also exhibits high ammonia nitrogen pollution characteristics, which undoubtedly increases the difficulty of printing and dyeing wastewater treatment. For example, a printing and dyeing wastewater treatment demonstration project completed by a carpet industry park in Jiangxi Province, completed by Donghua University (formerly China Textile University). Due to the use of ammonia pH buffering agent in the discharged printing and dyeing wastewater, the ammonia nitrogen in the wastewater can reach 100 ~ 200 mg. / L. In addition, in the treatment of nylon production wastewater, it was found that the wastewater discharged during the production of nylon contains a large amount of caprolactam monomer. Under aerobic conditions, caprolactam will be degraded to CO2, NH3 and H2O, which will greatly increase the ammonia nitrogen in the wastewater.
At present, due to the large differences in the products and raw materials of various printing and dyeing enterprises, the micro-components in the wastewater are not the same. Sometimes, although the COD of the wastewater discharged by different printing and dyeing enterprises is not much different, the treatment shown by the same treatment process The effectiveness is different. Therefore, "specific problems and specific analysis" must be carried out in the process of engineering design, operation and commissioning, and detailed preliminary scientific experiments and optimization of design and operation parameters according to printing and dyeing wastewater discharged from different enterprises can achieve the optimal treatment effect. This is the current difficulty in the treatment of printing and dyeing wastewater, and it is also the charm of attracting scientific and technological workers to continuously tackle new technologies in printing and dyeing wastewater treatment.
COD source of printing and dyeing wastewater
Many previous studies have focused on the treatment of printing and dyeing wastewater mainly in the removal of dyes. In fact, the contribution to the COD of printing and dyeing wastewater does not mainly come from dyes, because more than 90% of the dyes have been colored during the printing and dyeing process, and less than 10%. % Of the dye is discharged into the waste water, while a large amount of auxiliary agent enters the waste water during the printing and dyeing process. Many years of engineering practice and research conducted by Donghua University have also proved that the most important contribution to COD is the additive added in the printing and dyeing process. In bioengineering engineering design, priority should be given to the biodegradability of the additive. The generation of chromaticity of printing and dyeing wastewater plays a greater role. Therefore, analysis of COD sources and main components of pollutants in wastewater has an important guiding role in the selection of printing and dyeing wastewater treatment processes.
2. Status and problems of printing and dyeing wastewater treatment technology
Main processing technology and existing problems
At present, the physical, chemical and biological methods are mainly used for the treatment of printing and dyeing wastewater.
Among physical methods, physical adsorption methods and membrane separation methods are most commonly used. Physical adsorption method often uses activated carbon to adsorb water-soluble dyes, but activated carbon is easy to reach saturation and needs to be regenerated, and the regeneration cost is high. Therefore, this method is generally suitable for advanced treatment or wastewater with low concentration and small amount of water. The membrane separation method uses semi-permeable membranes with different pore sizes to filter and separate mixtures with different particle sizes. This method has stable water output and good effect, but the reuse rate of the separation membrane is low and the cost of the membrane is high. It is difficult to spread technology on a large scale.
Chemical methods mainly include chemical coagulation, ozone oxidation and photocatalytic oxidation. The chemical coagulation method relies on the intermolecular interaction to make small molecular suspensions, colloidal materials, etc. in the wastewater to form large molecular particles, which are then removed by precipitation or air flotation. The coagulation method has low treatment cost and simple operation, and is widely used in the current printing and dyeing wastewater treatment process. However, this method requires secondary treatment of sludge and has a poor decolorization effect on dyes with high water solubility. Ozone oxidation method has great advantages in treating wastewater chroma and reducing COD, but the cost of ozone generator is higher and the operation and management requirements are strict, making the effect of ozone oxidation method unstable in practical application. The photocatalytic oxidation method uses photocatalysts to generate free radicals to oxidize organic matter in wastewater into carbon dioxide and water. The current low utilization of sunlight by photocatalysts limits its application in the treatment of printing and dyeing wastewater.
Biological method is to remove organic pollutants in wastewater through the growth and metabolism of microorganisms. Due to the poor biodegradability of printing and dyeing wastewater, it is difficult to meet the discharge requirements by using biological method to treat printing and dyeing wastewater alone.
In practical applications, considering the water quality characteristics of printing and dyeing wastewater, physical, chemical and biological methods are usually used in combination to achieve the discharge of printing and dyeing wastewater.
Research and application of printing and dyeing wastewater treatment technology are disconnected
The treatment of printing and dyeing wastewater is an "ancient" subject, but new problems are constantly emerging in different periods. In China, the textile industry was in its infancy in the early days of the founding of the People ’s Republic of China. Most of the auxiliaries added during the printing and dyeing process were natural easily biodegradable substances (such as starch). Printing and dyeing wastewater could be discharged through biochemical or biochemical + physical and chemical processes. However, with the continuous improvement of the level of textile printing and dyeing technology and the increased demand for high-end printing and dyeing products, the use of refractory industrial synthetic dyes and auxiliaries has increased, which has significantly reduced the biodegradability of printing and dyeing wastewater, and greatly increased the difficulty of treatment. Sometimes it is difficult to meet the emission standard requirements only through the biochemical or biochemical + physical and chemical process route. In response to this new problem, in recent years, a large number of basic researches on the application of new printing and dyeing wastewater treatment technologies have been carried out at home and abroad, including advanced oxidation and membrane separation technologies. From the perspective of technology alone, these high-end technologies can completely solve the problem of removal of difficult-to-degrade COD in printing and dyeing wastewater, but in fact, most printing and dyeing enterprises have the characteristics of "small profits, small profits and quick turnover". The application of high-end technologies will inevitably bring investment and operating costs. The increasing problem is not applicable to printing and dyeing enterprises with low profit margins. Therefore, it is imperative to explore low-cost, efficient printing and dyeing wastewater treatment technology and put it into practice.
Problems caused by "raising bids" and "reuse"
At present, in order to achieve national energy-saving and emission reduction requirements, the printing and dyeing wastewater discharge standards have been greatly improved, and have generally been upgraded to Level B or even Level 1 in the "Pollution Discharge Standards for Urban Sewage Treatment Plants" (GB 18918-2002) in Jiangsu and Zhejiang regions. A standard; In order to control the total amount of sewage, the reuse rate of wastewater has gradually become a rigid requirement. In some areas, the reuse rate of printing and dyeing wastewater has reached 50% or more. After the revision of the 2012 Water Pollutant Discharge Standard for Textile Dyeing and Finishing Industry (GB 4287-2012), each emission limit was tightened on the original standard (1992 edition). For example, the 1992 version of the standard stipulated that the emission limits of COD, BOD5, and NH3-N were 180, 60, and 25 mg / L, respectively, and there was no clear requirement for total nitrogen and total phosphorus; and the 2012 version of COD, BOD5, and NH3-N emissions The limits were increased to 100, 25, and 12 mg / L, and the emission limits for total nitrogen and total phosphorus were clearly required to be 20 and 1.0 mg / L.
Discharge standardization and reuse have a direct and far-reaching impact on the treatment concept and technological innovation of printing and dyeing wastewater, which not only requires high-level discharge of treated wastewater, but also more importantly requires in-depth research on printing and dyeing wastewater treatment and reuse technologies and solutions. Printing and dyeing wastewater treatment and reuse are different from the reuse of water in the building. In addition to the consideration of flushing, car washing, greening and other links in the factory area, the reuse needs to involve production processes such as rinsing and dyeing with large amounts of water. For desizing, scouring, rinsing and other links with lower water quality requirements, the use of secondary biochemical treatment of printing and dyeing wastewater can meet the requirements; however, higher quality requirements such as dyeing, printing and bleaching often require higher quality. Reclaimed water, and this part of recycled water can only meet the requirements through high-end treatment technology. However, the reality is still restricted by the cost and cost of recycling, which has led to the wastewater reuse rate in the printing and dyeing industry being kept at a low level of less than 10%. For example: In the developed area of China ’s textile printing and dyeing industry—Shengze Town, Wujiang, the local environmental protection bureau surveys and analyzes that the acceptable recycling cost for local printing and dyeing enterprises is only 0.5 yuan / m3, but the current high-end treatments such as advanced oxidation and membrane separation The processing costs of technology are all above this level.
Control of total sewage discharge makes wastewater treatment more difficult
The implementation of total pollution control of printing and dyeing enterprises can effectively protect the environmental capacity, but it is a great challenge for the development and survival of current printing and dyeing enterprises. At present, if the printing and dyeing enterprises want to expand the reproduction without touching the bottom line of the total pollutant discharge, they must implement the source pollution control and end treatment of wastewater at a higher standard. On the one hand, companies must pay attention to clean production and eliminate backward process equipment with large pollution production; on the other hand, they must increase the treatment of printing and dyeing wastewater to achieve high standards of discharge and even reuse after wastewater treatment. In fact, the current level of technology and equipment of printing and dyeing enterprises in China lags behind developed countries. Anhydrous dyeing technology only stays at the laboratory research level. Most printing and dyeing enterprises still use the traditional process of large amount of wastewater. Due to funding reasons, enterprises implement clean production. The road is still long. In addition, most of the current printing and dyeing enterprises are private enterprises, and it is difficult to focus on building a low-cost, efficient terminal treatment technology R & D and research platform, and the capital investment is still insufficient. As a result, there is still a lack of both technology and economic key wastewater treatment reuse technologies.
3. Challenges and opportunities for future printing and dyeing wastewater treatment
(1) In response to the "standard increase" and "reuse", another path is to develop and apply efficient and low-consumption wastewater treatment and reuse technologies.
Facing the reality and characteristics of low profitability of the printing and dyeing industry, we should develop and apply low-cost, non-degradable printing and dyeing wastewater treatment technology in a targeted manner. In this regard, Donghua University has conducted some useful explorations and practices in recent years. For example, the introduction of low-cost, easy-to-manage ecological treatment technologies in the treatment and reuse of printing and dyeing wastewater. Although traditional views believe that ecological treatment technologies such as artificial wetlands are suitable for the restoration of low-COD, easily degradable domestic sewage and surface water bodies, It is still feasible to apply artificial wetland technology for treatment after a certain biochemical pretreatment. Donghua University applied the hydrolytic acidification-aerobic-artificial wetland combination technology to establish a demonstration project for the ecological treatment and reuse of printing and dyeing wastewater in a Jiangxi Carpet Industry Park Co., Ltd. The total cost of biochemical and ecological process water treatment was only about 1.0 yuan / m3. The effluent quality of the treated water met the water requirements of the production process in the plant area, and achieved "zero discharge" of wastewater.
Innovate and improve the existing commonly used wastewater treatment technology, and it is also a more feasible method for the treatment of printing and dyeing wastewater. Donghua University used low-resistance, cheap non-woven fabrics or woven fabrics to replace the organic membranes in the membrane bioreactor (MBR), which greatly reduced the one-time investment of the membrane unit, and could achieve complete gravity filtration with outstanding energy-saving effects. In addition, in order to solve the problem that the conventional process is difficult to achieve the standard discharge of printing and dyeing wastewater, a multi-metal catalytic reduction technology has been developed to deeply treat the effluent of the conventional process. At present, this technology has been obtained in laboratory research and pilot test research of a wastewater treatment plant in Xiaoshan. Satisfactory effect, and the processing cost is only about 0.5 yuan / m3, the technology is expected to further increase the scale of the test and a wide range of applications.
Liu Weijing of Nanjing University of Science and Technology adopted the combined process of "intensified anaerobic hydrolysis + A / O (PACT) + coagulation sedimentation filtration" to treat comprehensive wastewater in industrial parks dominated by textile printing and dyeing enterprises. The results showed that the total COD removal rate of the system reached 93.2%. The effluent water quality meets the requirements of Jiangsu Province's "Main Water Pollution Emission Limits for Urban Sewage Treatment Plants and Key Industrial Industries in the Taihu Region". Tongji University Architectural Design and Research Institute adopts ozone-biological filter, ozone-biological carbon pool-double-membrane treatment process to produce different quality recycled water, and achieves the purpose of quality separation and reuse, ensuring the quality of recycled water and the safety of project operation And feasibility, while reducing processing costs.
In addition, some new technologies are also being tried and applied in the process of printing and dyeing wastewater treatment. KR Parmar et al. Used acetone to reduce graphene to produce acetone-reducible graphene (ARGO), and then modified the surface of Fe3O4 with ARGO to prepare magnetic reducing graphene. This material has a good dye adsorption effect, and the magnetic properties Materials can be recycled by controlling the magnetic field, which effectively reduces the cost of materials. In addition, TiO2 photocatalytic oxidation, cationic flocculants, nano-iron-enhanced flocculation / sand filtration / ultrafiltration technologies have also been applied and gradually developed in the treatment of printing and dyeing wastewater.
(2) To cope with "total pollution control" and explore economically feasible system reuse engineering schemes.
The printing and dyeing process involves multiple links, and each link has different requirements for water and water quality. Therefore, it is necessary to comprehensively analyze the wastewater reuse plan from the perspective of "quality reuse" in order to reduce the reuse cost to the greatest extent, which is different Process links use different degrees of wastewater treatment. For example, in the high-quality printing, dyeing, bleaching, etc., the secondary biochemical effluent needs to be desalted after the nanofiltration (NF) or reverse osmosis (RO) in-depth treatment; while desizing, scouring, rinsing, etc. The lower requirement is to use biochemical effluent from printing and dyeing wastewater. Donghua University followed the idea of "quality and reuse" in the design and treatment of printing and dyeing wastewater treatment of a certain printing and dyeing technology company in Shandong with a scale of 20 000 m3 / d. Intensive treatment and reuse can reduce the comprehensive cost of printing and dyeing wastewater treatment and reuse to about 1.6 yuan / m3. In short, the treatment and reuse of printing and dyeing wastewater should be considered on the "global" basis of the production process of the printing and dyeing plant, and the comprehensive reuse engineering scheme should be determined from the perspective of "system engineering", so as to achieve the purpose of optimal technical and economical efficiency.
(3) Innovate the concept and enable enterprises to accept "recycled water" for printing and dyeing wastewater treatment.
In addition to technical and economic reasons, perception is also an important factor restricting the use of recycled water. After investigations on a number of printing and dyeing enterprises, it was found that although the quality of the reused water for the deep treatment of printing and dyeing wastewater has reached the process requirements, and the dyeing experiments show that the quality of the cloth color also meets the requirements, the company always feels insecure about using the recycled water and is very worried The use of recycled water will affect the sales of the product. In order to change the concept, in addition to fully demonstrating technology, government propaganda, systems, and policies must be induced and promoted to enable enterprises to conceptually accept "recycled water."
Related equipment: printing and dyeing wastewater treatment equipment zl-yrws001