• The Source and Harmfulness of Coating Wastewater

The painting process of iron parts: pre degreasing degreasing water washing water washing surface adjustment phosphating water washing water washing water washing drying spray painting drying.

The coating process for plastic parts: degreasing - washing - washing - washing - interface activation - drying - spray painting - drying - spraying conductive agent - electrostatic spray painting - drying.

The degreased washing water contains many surfactants and emulsified oil stains. The COD in the water is about 700mg/L, and the BOD is about 200mg/L. If this water is not treated, it is directly discharged into rivers. When the organic matter in the wastewater decomposes in the water, it consumes a large amount of dissolved oxygen, which disrupts the oxygen balance in the water and produces a foul odor.

The water washed after phosphating contains heavy metals such as nickel ions (Ni X) and zinc ions (Zn X) that exceed the emission standards. It is well known that nickel ion is a carcinogen; Excessive zinc has a significant toxic effect on aquatic organisms. During the painting process, paint mist will be generated.

To achieve normal production, it is necessary to remove the abandoned paint mist from the spray booth. A common and effective method is to set up a water curtain on the side of the spray line (i.e. the population of the exhaust duct), allowing the water curtain to absorb most of the paint mist. Waste gas that is not absorbed by the water curtain can be treated using the method of treating waste gas. There are various types of paints, and the formulas used in the painting workshop are confidential. However, regardless of any paint, the paint and its organic solvents are toxic, and even highly toxic.

Organic solvents usually have the following types: aromatic type: vinegar such as toluene, xylene, phenylethane, ketones such as acetic acid acetic acid acetic acid acetic acid acetic acid butyl acetate, alcohols such as acetone, cyclohexanone, etc. Water curtain agents such as ethanol, butanol, isopropanol, etc. are generally composed of caustic soda and alkali resistant composite organic compounds that can absorb paint mist. After the water curtain absorbs paint mist, the composition of the water becomes very complex, with high toxicity and high organic content. According to analysis, its CODcr is often at several thousand mg/L, sometimes as high as 13000mg/L.

Although water curtain water can usually be recycled after removing scum (or sediment), there are inevitably two situations: one is that a portion of the water curtain water is discharged into wastewater, and sufficient fresh water and water curtain agent are added; The second is that the water curtain is completely updated after a certain period of cyclic use. If this high content and highly toxic water is discharged directly into rivers (lakes) without treatment, the harm it brings to humans is unimaginable, so it must be treated.

• Introduction to Coating Wastewater Treatment Process

2.1 Introduction to Painting Process

The painting process generally consists of three basic processes: surface treatment before painting, painting, and drying.

Pre coating surface treatment, also known as pre-treatment technology, is the foundation of the entire coating process, mainly including surface cleaning (rust removal, degreasing, etc.) and phosphating treatment. Degreasing mainly relies on the saponification, wetting, emulsification, penetration, separation, dispersion, and solubilization of degreasing agents on oil and dirt to remove oil and grease from the surface of machine components or iron products. Degreasing agents mainly include alkaline degreasing agents, cleaning aids, additives, surfactants, defoamers, etc;

Phosphating treatment is the formation of a non-metallic, non-conductive, porous phosphate film on the surface of a metal through chemical reactions. The phosphate film can significantly improve the adhesion, corrosion resistance, and water resistance of the coating.

Generally, thin film zinc salt is used for rapid phosphating treatment of car bodies and carriages. After phosphating treatment, 2-3 more cleaning cycles are required. After cleaning, the coating electrophoresis process is carried out. After electrophoresis, 2-3 recovery water washing is carried out with ultrafiltration solution, and then deionized water washing is used. In addition, sedans and light trucks with decorative requirements are generally coated with intermediate coatings using the electrostatic coating method; The topcoat is generally made of melamine alkyd resin enamel, using automatic spraying or electrostatic spraying.

2.2 Processing Technology and Its Research Progress

At present, there are multiple technologies that should be used for comprehensive treatment of coating wastewater or for further treatment of a certain component. The scope of research and engineering practice involves various treatment methods such as physical, chemical, and biological methods.

2.2.1 Physicochemical treatment methods

The physicochemical method generally adopts two-stage coagulation sedimentation, which involves adding coagulants to the wastewater to form large particle agglomerates of pollutants, which are removed through sedimentation and filtration. The physicochemical method has simple principles, saves investment, and occupies less land, and can generally be used in sewage treatment projects with low discharge requirements. However, using physical and chemical treatment alone is difficult to achieve high water quality requirements.

2.2.2 Biological treatment methods

The biological treatment method is to use microorganisms to degrade organic matter in water. The advantages of this method are low operating cost, high removal rate of organic matter, and low energy consumption during the treatment process. However, microorganisms have high requirements for environmental conditions and have a long processing cycle. During the production process of Jiangling Group's coating workshop, a large amount of coating wastewater is generated, which contains toxic substances such as emulsified oil, surfactants, water-soluble resins, pigments, and paints.

2.2.3 Physicochemical and biochemical treatment methods:

Due to the complex composition, large fluctuations in water volume, and irregular discharge characteristics of automotive coating wastewater, when using a single physicochemical or biological method to treat wastewater, there are characteristics such as unstable effluent and high treatment costs. With the gradual improvement of wastewater discharge standards, physicochemical and biochemical methods have replaced a single treatment method and become the main method for treating automotive coating wastewater in the future. This method has the characteristics of stable treatment effect, low operating cost, and convenient operation and management. It can effectively remove pollutants from automotive coating wastewater and has good economic and social benefits.

2.3 Other research progress

The composition of automotive coating wastewater is complex, and the treatment involves a wide range of aspects. Therefore, in order to better treat coating wastewater, in addition to the research and practice of traditional physicochemical, biological, and physicochemical biological methods, many scholars have also conducted extensive research on aspects such as nickel removal from coating wastewater, operation and maintenance of treatment equipment, and wastewater recycling. Automotive coating wastewater contains high concentrations of nickel. The treatment process of automobile coating wastewater generally adopts coagulation sedimentation and biochemical treatment, often neglecting the treatment of nickel. Nickel is a type of pollutant strictly controlled by the country, and the treatment difficulty of nickel containing wastewater is much greater than that of wastewater containing other metals. Studying the process parameters for removing nickel from automotive coating wastewater has practical significance.

• Conclusion and Prospect of Coating Wastewater Treatment

The wastewater generated in the automotive coating production process contains various polymer resins, surfactants, emulsified oils, and organic solvents, which have the characteristics of large fluctuations in water volume and quality, complex composition, high COD content, but poor biodegradability. At present, the commonly used methods for treating coating wastewater include physical and chemical methods, biological methods, and physical and chemical methods. Multiple methods and treatment methods are combined to achieve comprehensive treatment. For coating wastewater with complex components and high pollutant concentration, ideal treatment results can be achieved.

From the perspective of sustainable development and clean production, it is necessary to improve the coating production process, improve coating materials, improve coating technology, and strengthen coating production management in order to reduce pollutant emissions.