Views: 106 Author: Site Editor Publish Time: 2024-06-12 Origin: Site
The automobile painting process is an indispensable part of every automobile production process.
After the final production, the automobile is assembled from pieces of exposed steel plates. There are two main purposes for painting the automobile after production. One is to beautify the appearance of the car itself, and the other is to prevent the steel from being corroded.
Industrial wastewater generated in automobile painting includes degreasing wastewater, phosphating wastewater, electrophoresis wastewater, and painting wastewater. This wastewater contains pollutants such as heavy metal ions, surfactants, paint particles, and phosphates, which are challenging to treat in high concentrations.
Pretreating the wastewater based on its quality can effectively remove impurities such as phosphates, heavy metal ions, suspended solids, and oil substances, reducing the load of subsequent treatment units and improving overall efficiency.
This article reviews recent research and practices in automobile painting wastewater treatment, aiming to summarize the advantages and disadvantages of pretreatment and main treatment processes, providing valuable insights for professionals in related industries.
Table of contents（Click to go to where you want to see）
4. Characteristics of painting wastewater
5. The main process of wastewater treatment
5.1 Physical and chemical system processing
5.2 Biochemical Systems Processing (Biochemistry)
6. Main treatment technologies for automobile painting wastewater
7. Main reaction tanks and their functions in wastewater treatment systems
7.1 Principle and function of phosphating neutralization coagulation reaction tank
7.2 Principle and function of electrophoresis neutralization coagulation reaction tank
7.3 The principle and function of Lamela sedimentation tank
7.4 Function and principle of vertical (vertical) flow sedimentation tank
8. Important control parameters of wastewater treatment system
9. New technology for treating nickel-containing phosphating wastewater from coating
10. Painting wastewater treatment case
The process of automobile painting is very complicated, including electrophoretic painting and polishing, color paint and varnish brushing, baking, spot repair, assembly and repair, etc.
The process starts with weak alkali leaching. After weak alkali leaching, it is washed with clean water. After washing the car body with clean water, there are often residual oil stains on the surface of the car body. At this time, the spraying method is used to remove the residual oil stains on the surface. After removing the residual oil stains on the surface, the car body is washed with water, and then phosphating is performed again after washing, and then washing again, and then washing with pure water, followed by electrophoresis, spraying, deionized water spraying, and finally painting. The entire automobile painting process ends here.
Automobile painting is an essential part of automobile production and has three main functions for the finished automobile:
making the automobile's appearance more aesthetically pleasing;
preventing the metal components in the automobile from rusting and damaging, thereby extending the service life of the car;
providing different body colors to meet the personalized needs of consumers.
The primary source of painting wastewater is generally in the pre-treatment process of the vehicle body. Water is required after the simple decoration is completed for the pre-degreasing, degreasing, phosphating, and passivation processes of the vehicle body. Water plays an irreplaceable role in these processes. Therefore, the painting wastewater generated in these links is the link that produces the most wastewater.
Degreasing wastewater
The degreasing area is the first process for welded car bodies to enter the paint shop. The commonly used degreasing tank liquid is an aqueous solution containing surfactants and alkali salts. When the degreasing area decreases the car body, low-flow, low-concentration degreasing wastewater will be continuously discharged. When the degreasing liquid has been used for a period of time, high-flow, high-concentration degreasing wastewater will be discharged during the shutdown period. Degreasing wastewater is alkaline and mainly contains pollutants such as oils and surfactants.
Phosphate wastewater
The phosphating area is where the metal surface of the car body undergoes a phosphating reaction after being degreased and cleaned in the degreasing area. During the production process, low-flow, low-concentration phosphating wastewater will be continuously discharged, and when the phosphating liquid has been used for a period of time, high-flow, high-concentration phosphating sewage will be discharged during the shutdown period. Phosphating wastewater is acidic and contains heavy metal ions such as zinc, manganese, nickel, and aluminum, as well as non-metallic ions such as fluoride and phosphate.
Passivation wastewater
The primary purpose of the passivation process is to passivate the defective parts of the car body surface where the phosphate film has not been formed, to improve the corrosion resistance of the parts after combining with the subsequent coating, and to make the surface smooth for the subsequent electrophoresis. The passivation wastewater is acidic and mainly comes from the passivation tank waste and cleaning liquid.
Electrophoresis wastewater
The most common electrophoretic paint is water-based electrophoretic paint, mainly composed of deionized water, resin, color paste, and solvent. Electrophoretic wastewater primarily comprises high molecular weight water-soluble resin substances with complex composition.
Other wastewater
Other common wastewaters in paint workshops are mainly workshop air-conditioning wastewater, pure water cleaning reverse osmosis membrane acid and alkali wastewater, etc., which are uniformly collected in the wastewater treatment system's acid and alkali mixed storage tank.
| Wastewater Type | Wastewater Type | Characteristic pollutants |
| Degreasing wastewater | Wastewater generated by using hot alkali and organic solvent to clean the car body to be painted, and removing grease, iron filings, dust particles, etc. on the surface of the car body | Suspended solids (SS), polymerized alkaline salts, surfactants, oils and fats |
| Electrophoresis wastewater | Wastewater generated by the electrophoresis process | Water-soluble resins, SS, COD (chemical oxygen demand) |
| Painting wastewater | Wastewater formed by capturing the floating paint mist in the spray room and dissolving it in water | COD, SS, paint mist particles |
| Phosphating wastewater | Wastewater generated when the workpiece is immersed in the phosphating solution and a crystalline phosphate film is formed on the surface | Phosphates, SS, metal ions |
Table 1 Analysis of pollution source of coating process
Due to the different coating processes and coatings used, the quality of wastewater from each process varies greatly. This type of wastewater usually has the following characteristics:
A wide variety of ingredients
There are many kinds of chemical agents added in the painting process of automobiles, and their acidity and alkalinity are different, resulting in different quality and volume of various wastewaters, and the content of pollutants in each type of wastewater will fluctuate with the different proportions of materials.
The wastewater generated by the painting process contains various forms of toxic and harmful substances such as Ni2+, Cu2+, Zn2+ and its compounds, VOCs, paint particles, etc. These pollutants will cause potential harm to the receiving water and soil environment. According to literature research, the water quality parameters and ranges of painting wastewater are obtained, as shown in Table 2.
Poor biochemical
The content of organic matter and heavy metals in painting wastewater is relatively high, so it cannot be directly treated by biochemical treatment. It needs to be treated by physical and chemical treatment to reduce the chemical oxygen demand (COD) and heavy metal content before it can enter the biochemical treatment.
The electrophoresis and painting processes produce a large amount of organic pollutants. When using biological treatment processes, the biotoxic substances in the wastewater may inhibit the activity of microorganisms or even cause microbial poisoning and death, affecting the treatment effect.
Therefore, it is necessary to select appropriate pretreatment processes for different water qualities to share the load of subsequent biological or advanced treatment.
Large discharge volume, water quality is prone to change
There is no regularity in the wastewater generated by the coating process during production. Different processes produce pollutants of various concentrations and types. The wastewater formed after each process is mixed has the characteristics of a large fluctuation range of water volume and unstable water quality.
Given the large amount of toxic and harmful substances in coating wastewater, such as organic matter, heavy metals, grease, etc., advanced technologies such as membrane separation, advanced oxidation, electrodialysis, and electrolysis have been increasingly widely used in treating such sewage. Most coating workshops in developed countries such as Europe and the United States first pre-treat the wastewater from each process by quality separation and then use reverse osmosis electrodialysis and other technologies for deep treatment. The effluent water quality is good, and most can be reused.
The author believes that according to the differences in the use of coatings and coating processes by automobile companies, using new combined processes to improve the quality of treated water can achieve large-scale or complete reuse of wastewater.
| Index | Numeric |
| pH | 6~11 |
| ρ（COD）/（mg·L-1） | 1 000~16 200 |
| ρ[ 5-day biochemical oxygen demand（BOD5）]/（mg·L-1) | 100~350 |
| ρ(SS)/(mg·L-1) | 400~600 |
| ρ（petroleum class）/（mg·L-1） | 30~85 |
| ρ（PO43-）/（mg·L-1） | 20~50 |
| ρ（Zn2+）/（mg·L-1） | 5~150 |
| ρ（Cu2+）/（mg·L-1） | 0. 05~1. 3 |
| ρ（Cd2+）/（mg·L-1） | 0. 05~3 |
| ρ（Ni2+）/（mg·L-1） | 6~30 |
Table 2 Water quality parameters and scope of coating wastewater
Physical and chemical system processing (physical and chemical)
Anodizing wastewater is waste generated during the metal surface treatment process in the electroplating industry. Its hazards are mainly reflected in the following aspects:
The main objects removed by the physicochemical system are delicate suspended matter and colloidal particles, various high molecular organic matter, and metal substances in the wastewater, using coagulation and sedimentation technology.
Coagulation and sedimentation are to add appropriate coagulants into the wastewater, and after sufficient stirring, mixing, and reaction, the tiny suspended particles and colloidal particles in the wastewater will coagulate with each other to form floccules with a particle size of > 20 μm and easy to settle, and then separate the mud and water through natural sedimentation.
Biochemical Systems Processing (Biochemistry)
After physical and chemical treatment, the wastewater enters the biochemical reaction tank through the mixed wastewater intermediate water tank and is aerated by a fan. Aerobic bacteria decompose most of the organic matter into inorganic matter. The decomposed wastewater is discharged to the biochemical sedimentation tank, and then the mud and water are separated by natural sedimentation.
The supernatant enters the effluent monitoring water tank through the water collection tank, and part of the sludge flows back to the biochemical reaction tank to maintain the sludge concentration in the biochemical reaction tank. The sludge pump discharges the remaining sludge to the sludge concentration tank for further concentration. The filter press filters the concentrated sludge to form a mud cake (water content is 65% ~ 75%), treated as hazardous waste by a qualified third party.
Since the pollutants in painting wastewater vary, including surface cleaners, resins, petroleum, paint, etc., the COD is relatively high, and some wastewater contains nickel. If it is not adequately treated and discharged into natural water bodies, it will significantly impact the ecological environment. Standard automobile painting wastewater treatment technologies are as follows.
Coagulation and sedimentation
Coagulation uses the reaction mechanism of chemical agents in wastewater to promote the continuous collision of tiny colloid particles in the water with suspended impurities, resulting in polymerization and larger flocs. The flocs continue to increase in size and produce precipitation when they reach a specific limit, achieving the purpose of removing pollutants.
In electrophoresis and phosphating wastewater, the concentration of delicate suspended matter and colloids is very high, and coagulation precipitation can separate most of the pollutants.
Air flotation technology
Dissolved Air Flotation technology uses a particular flotation device to pass air through a bubble generator to generate many fine bubbles, which are mixed into the water and attached to the pollutant particles, resulting in increased buoyancy. Finally, the particles float to the water surface through buoyancy to achieve solid-liquid separation.
Paint molecules in painting wastewater react with coagulants to generate viscous paint residues, which gradually float to the water surface after being attached by bubbles and can be removed by scrapers. Grease in degreasing wastewater and paint in electrophoresis wastewater can remove most of the pollutants through flotation technology, reducing the subsequent treatment load.
Membrane treatment technology
The main working principle of membrane treatment technology is to use gradient pressure as the driving force and utilize the permeability of a specific membrane to separate ions and fine suspended solids in water. It is one of the standard treatment methods for coating wastewater.
Generally, it can be divided into six categories: microfiltration membrane technology, ultrafiltration membrane technology, nanofiltration membrane technology, reverse osmosis membrane technology, dialysis membrane technology, and forward osmosis membrane technology.
In the deep treatment of wastewater, membrane technology is widely used, and different membrane materials have different separation effects, concentrations, and purification of pollutants.
In general, the treatment efficiency is still relatively high, but the maintenance and replacement costs of the membrane are not low.
Biological treatment technology
Biotechnology mainly uses microorganisms and various bacterial fluids in wastewater to metabolize organic and inorganic matter and degrade water pollutants. There are primarily aerobic and anaerobic treatments.
However, because wastewater contains some toxic and harmful substances, it will inhibit the normal reproduction and growth of aerobic microorganisms, and the biodegradability of wastewater is not good.
Therefore, the wastewater can be mixed evenly first, hydrolysis and acidification can be increased, and anaerobic treatment can be carried out. After biodegradability is improved, the effluent efficiency of aerobic therapy will be significantly improved.
There are many types of pollutants in painting wastewater, and every kind of wastewater has a matching treatment technology.
Therefore, a combination of methods is usually required to treat painting wastewater. After the wastewater is pretreated, the biodegradable part is further treated by biological processes so that the treated wastewater can be recycled or discharged to meet the discharge requirements.
According to relevant information and test data, the optimal reaction pH ranges of various ions are Ni2+≥9.5, Zn2+=8~11, Mn2+≥9, and PO43-≥8.5, so the pH determined by this phosphating wastewater treatment system is between 9.5 and 11.
The phosphating neutralization coagulation reaction tank adjusts the influent pH between 9.5 and 10.5 by adding calcium hydroxide/sodium hydroxide and removes heavy metals by forming precipitation through the reaction of hydroxide ions with heavy metals, then adds PAC (poly aluminum chloride) for coagulation-to-form-small-particle-flocs-and-removes-PO43-ions-in the sewage through aluminum salt, and then adds PAM coagulant to coagulate the small particle flocs into large particle flocs, and then precipitates through the Lamellar sedimentation tank.
The removal method is a chemical removal method. By adding Ca2+ and aluminum salt, phosphate radicals react to form phosphate precipitates that are insoluble in water, separating them through sedimentation tank precipitation. The chemical reaction equation is:
5 Ca2+ + 3 PO43- + OH-= Ca5
(OH)(PO4)3↓, Al3+ + PO43-= AlPO4.
Metal ions react with OH-ions to form hydroxides that are insoluble in water. Then, the metal ions are transferred from the water to the sludge through precipitation and treated. The chemical reaction equation is:
Ni2+ + OH- → Ni(OH)2↓,
Zn2+ + OH- → Zn(OH)2↓,
Al3+ + OH- → Al(OH)3↓,
Mn2+ + OH- → Mn(OH)2↓.
The electrophoresis neutralization and coagulation reaction tank adds calcium hydroxide to the electrophoresis wastewater to make the wastewater pH> 10 and continuously stir the reaction for more than 10 min to denature the high molecular substances in the wastewater and reduce its bonding ability. Sodium hydroxide is added in the subsequent reaction cell to increase the pH adjustment accuracy and reaction time.
Finally, flocculants such as PAC and PAM are added. After the flocculation reaction produces large flocs, the overflow enters the vertical flow pre-sedimentation tank. The supernatant returns to the wastewater storage tank through the overflow. The sedimentation and concentration produce wastewater with a high mud content, which is further separated from the mud and water after being sent to the concentration sedimentation tank.
The clear liquid wastewater of the phosphating wastewater coagulation reaction tank and the mixed wastewater coagulation reaction tank are precipitated in a Lamella Plate Clarifier (inclined plate sedimentation tank).
The Lamela sedimentation tank is also called an inclined plate sedimentation tank. The wastewater flows from the bottom to the upper side between the multiple layers of inclined plates. During this process, the settled sludge moves and separates between the inclined plates on both sides, shortening the particle settling distance. The inclined plate sedimentation tank not only shortens the sedimentation time but also increases the sedimentation area of the sedimentation tank, which improves the wastewater sedimentation treatment efficiency.
Electrophoresis wastewater contains a large amount of high molecular organic matter and has high viscosity. If a Lamela sedimentation tank is used, the sludge will adhere to the inclined plate, resulting in a reduced sedimentation effect of the inclined plate sedimentation tank, which is extremely difficult to clean.
The inlet water flow of the vertical (straight) flow sedimentation tank is guided to the bottom by the central empty pipe so that the water flows from bottom to top and the water flows upward from the bottom.
Under gravity, the sediment is precipitated from top to bottom. The rising speed of the wastewater flow is similar to the sedimentation speed of the sediment in the wastewater so that a suspended layer is formed, which collides with the floccules that have not reacted with the flocculant flowing into the sedimentation tank. The flocculation reaction will continue, making the flocs larger and accelerating the sedimentation.
Important control parameters of wastewater treatment system
After coagulation and sedimentation treatment, the industrial wastewater effluent is alkaline and needs to be adjusted to pH before entering the biochemical system. Microorganisms in the wastewater biochemical treatment system are susceptible to the pH of the incoming water, and the impact of pH fluctuations on activated sludge is rapid and noticeable.
The activated sludge system also takes a long time to recover from inhibition. Therefore, during the reverse adjustment process, the acid dosage is controlled to stabilize the pH fluctuation and avoid large fluctuations in pH.
Since the temperature of wastewater produced by the coating workshop is close to room temperature, and each wastewater storage tank has the function of a regulating tank, it can effectively cool and pre-treat the medium and high-temperature water discharged from the front end.
The biochemical system is affected by temperature, mainly due to the temperature change caused by seasonal changes. 5.3 Sedimentation ratio (SV 30%) The sludge sedimentation ratio refers to the volume ratio of the precipitated sludge to the original mixed liquid when the sludge-water mixture in the biochemical reaction tank is left to stand in a measuring cylinder for 30 min, expressed in %.
After 30 min of static sedimentation, typical activated sludge can approach its standard density, and the average value is 20% ~ 30%. This indicator can relatively reflect the sludge concentration, coagulation, and sedimentation performance and serve as a reference indicator for controlling sludge discharge and early expansion.
The content of free oxygen in water is expressed as DO in mg/L. The dissolved oxygen in the aeration tank is usually controlled at 2-4 mg/L, which can not only meet the needs of aerobic microorganisms in the sludge for dissolved oxygen, but also avoid excessive aeration that causes the flocs to break up and accelerates the aging of the sludge.
Currently, the primary methods for treating heavy metal wastewater from surface treatment, such as coating phosphating in actual use, include chemical precipitation, membrane physical filtration, and electrochemical processes, among which chemical precipitation includes alkali precipitation, sulfur precipitation, and polymer chelation precipitation.
Among the many methods, chemical precipitation is the most widely used, and it is traditional, simple, and reliable. Suppose the wastewater treatment system using this method still has problems such as excessive heavy metals, laamountsount of sludge, and poor impact resistance. In that case, it is mostly not a problem of the chemical method itself but a failure to select a more reasonable process and a more appropriate chelating agent.
Using polymer chelating agents to treat heavy metal wastewater from coating phosphating is a new type of water treatment technology.
It has both a chemical coordination reaction and a physical adsorption process. The chelation effect of its group on heavy metal ions is much greater than the ionic bonding effect of hydroxide, and it has a good chelation flocculation effect. The chelation treatment project takes "alkaline precipitation pretreatment + chelating agent secondary precipitation + conventional filter + pH/ORP automatic control technology" as the main process line, which meets the requirements of the new emission standards and reduces the treatment cost. The system operates stably and has strong impact resistance.
There are many types of automobile painting wastewater, and the water quality is complex and fluctuates greatly, so it is difficult to treat.
According to the characteristics of painting wastewater, the wastewater treatment process needs to be targeted, and attention should be paid to the balance of water volume and water quality and pre-treatment of different qualities, which can prevent the impact on the wastewater treatment system and improve the stability of the operation of the wastewater treatment system.
