Views: 98 Author: Site Editor Publish Time: 2024-06-14 Origin: Site
| Wastewater Type | Painting wastewater treatment |
| Implementation time | 2021.6 |
| Processing scale | 200T/D |
| Project name | An example of a painting wastewater treatment project in an automobile manufacturing company |
| Product name | Lamella Plate Clarifier |
| Process |
Table of contents（Click to go to where you want to see）
2.1 Comprehensive wastewater treatment process
2.2 Phosphating wastewater treatment process
3. Wastewater treatment by quality
3.1 Electrophoresis wastewater pretreatment
3.2 Hydrogen peroxide wastewater pretreatment
3.3 Phosphate wastewater homogenization treatment
The painting wastewater discharged by a specific automobile manufacturing enterprise adopts the "pretreatment + comprehensive treatment" process according to the wastewater discharge situation and pollutant characteristics. According to the workshop wastewater discharge requirements, after fully meeting the standards, it enters the biochemical system of the factory area. It is further treated and discharged to the municipal sewage treatment plant.
According to the wastewater classification method described above, painting wastewater is divided into the following categories. The average daily wastewater discharge volume and water quality indicators of the workshop are shown in Table 1.
| Wastewater name | Water volume/（t·d-1） | SS/（mg·L-1) | COD/（mg·L-1） | TP /（mg·L-1） | Ni /（mg·L-1） | pH |
| Degreasing wastewater | 120 | 230 | 5000 | 180 | 12 | |
| Electrophoresis wastewater | 200 | 370 | 7000 | 70 | 5 | |
| Phosphate wastewater | 150 | 200 | 700 | 130 | 30 | 6 |
Table 1 Average water quantity and water quality indicators
The pollutant concentration of degreasing wastewater is high, and it is yellow and turbid. A layer of butter will adhere to the equipment and is floating on the upper layer. When the sludge is pressed, it will also stick to the filter cloth, blocking the filter holes and affecting the water permeability, resulting in prolonged pressing time and increased water content of the mud cake. The degreasing agent is relatively alkaline because much grease must be removed from the car body.
Because the degreasing liquid is highly alkaline and contains a variety of surfactants, the pH of the waste liquid produced is very high, reaching 13~14; the turbidity and suspended solids concentration are very high, and the oil content is significant, especially the degreasing waste liquid of the tank cleaning. The workshop is washed by high pressure. After flood water washing, the degreasing wastewater is discharged into the degreasing wastewater pool and enters the ultrafiltration oil removal device for pretreatment first.
Electrophoresis wastewater
Electrophoretic wastewater is wastewater generated when treating the surface of a car body. Impurities, various greases, rust, etc., on the surface of the car body need to be thoroughly removed before entering the subsequent treatment process. The wastewater mainly contains electrophoretic paint comprising polymer resins, neutralizers, organic solvents, and pigments. The COD and SS content is very high, the water quality is turbid, and the paint residue generated during the treatment process makes it easy to clog the pipeline.
Phosphate wastewater
Phosphating wastewater consists of pickling wastewater, phosphating wastewater, phosphating spray, flushing water, etc. Phosphate ions, copper, iron, lead, and other heavy metal ions are present in the wastewater, especially nickel ions, a pollutant containing many. Suppose the wastewater is not treated to meet the standards. In that case, the discharged wastewater will cause significant harm to the microorganisms in the natural water bodies and affect the sustainable development of the ecological environment.
Quality-based pretreatment is adopted according to the source, nature, and discharge requirements of wastewater.
Before comprehensive treatment, wastewater is collected and pretreated by quality, which can significantly reduce the treatment pressure of subsequent units.
Batch treatment, ultrafiltration oil removal, and other methods are pretreatment processes. According to the actual situation on site, wastewater treatment is divided into two lines: the comprehensive and phosphating wastewater treatment lines.
The electrophoresis wastewater with high pollutant concentration is pre-treated by batch physicochemical coagulation and then quantitatively added to the electrophoresis washing water and degreasing washing water with lower concentration according to a certain ratio with the degreasing wastewater after ultrafiltration oil removal to form comprehensive wastewater for treatment together. The comprehensive wastewater treatment process is shown in Figure 1, and the main structures of the extensive line and their functions are shown in Table 2.
Figure 1 Schematic diagram of comprehensive wastewater treatment process
| Structure name | Material | Volume/m3 | Main function |
| Comprehensive wastewater pool | Concrete | 230 | Collect various types of washing water and adjust water quality |
| Batch tank | pp | 15 | Pretreatment of electrophoresis wastewater |
| pH reaction tank 1 | Concrete | 6 | Add lime milk to remove phosphorus |
| pH reaction tank 2 | Concrete | 6 | Add NaOH to adjust pH |
| Concrete tank | Concrete | 10 | Add PAC, concrete consolidation |
| Flocculation tank | Concrete | 10 | Add PAM, flocculate into agglomerates |
| Pre-separation tank | Concrete | 90 | Primary sedimentation separation |
| Inclined plate sedimentation tank | Concrete | 118 | Secondary sedimentation and clarification |
| Sludge tank | Carbon steel | 8 | Storage of concentrated sludge |
Table 2 Main structures and their functions of the integrated line
As can be seen from Figure 1 and Table 2, various types of wastewater in the comprehensive wastewater pool are fully mixed and then pumped into reaction tank 1 by the lifting pump. After adding lime milk, they enter reaction tank 2, and sodium hydroxide liquid is added to raise the pH to 9.5~10.5. Then, they react with coagulants and are mixed and aggregated into fine flocs. Under the action of the flocculation tank PAM, a large number of alum flowers are formed. After flowing through the pre-separation tank, the flocs gradually settle, and the pollutants are separated from the clean water. Then, they flow into the inclined plate sedimentation tank, precipitate again, and then adjust the pH. The entire process treatment process is completed.
Phosphating wastewater treatment process
Phosphating wastewater adopts a separate sequencing batch chemical precipitation method. Most heavy metal ions, suspended solids, phosphates, and some insoluble organic matter can be effectively removed under alkaline conditions (pH around 10). The phosphating wastewater treatment process is shown in Figure 2, and the main structures of the phosphating line and their functions are shown in Table 3.
Figure 2 Schematic diagram of phosphating wastewater treatment process
As can be seen from Figure 2, the phosphating wastewater discharged from the workshop is lifted from the outdoor reserve tank to the buffer tank and then enters the batch treatment tank. It adopts a sequential batch continuous treatment method. First, ferric chloride solution and lime milk are added after sufficient reaction. Then, the sodium hydroxide solution controls the pH at about 10.5. After the reagent mixing reaction is completed, it enters the flocculation tank, and subsequent treatment unit, and the pollutants in the water are gradually precipitated and separated.
| Structure name | Material | Volume/m3 | Main function |
| Buffer tank | Stainless steel | 100 | Regulating phosphating wastewater |
| Batch tank | pp | 20 | Add ferric chloride solution, etc., and mix and react |
| Flocculation tank | pp | 10 | Add PAM to flocculate |
| Pre-separation tank | Carbon steel | 10 | Primary sedimentation separation |
| Sedimentation tank | Carbon steel | 10 | Secondary sedimentation and separation |
| Neutralization tank | pp | 10 | Adjust water quality and quantity |
| Post-neutralization tank | pp | 10 | Acid-base neutralization |
| Final discharge pond | pp | 10 | Effluent discharge |
Table 3 Main structures and functions of phosphating line
Wastewater treatment by quality
Electrophoresis wastewater pretreatment
The primary pollution indicator of comprehensive wastewater treatment in workshops is COD, which comes from high-concentration electrophoresis wastewater, including water-soluble resins, pigments, fillers, and cosolvents. The COD value can reach thousands or tens of thousands. The more fine slag in the wastewater, the higher the concentration of suspended solids and the greater the COD value.
As seen from Table 4, the COD concentration is positively correlated with the SS concentration within a specific range. It changes with the amount of coagulant added. Under certain conditions of lime milk addition, when the average amount of coagulant added reaches 2.5 L/t, the COD concentration does not drop significantly, slows down, and reaches the minimum value.
The SS concentration does not change much, the supernatant is clear, and most of the insoluble COD in the water has been removed.
| Coagulant dosage/(L·t-1) | Water output index | ||
| SS/（mg·L-1） | COD/（mg·L-1） | TP/（mg·L-1） | |
| 1.50 | 310 | 3200 | 6.6 |
| 1.75 | 276 | 1836 | 5.2 |
| 2.00 | 106 | 325 | 4.3 |
| 2.25 | 38 | 1189 | 3.1 |
| 2.50 | 8 | 953 | 1.4 |
| 2.75 | 7 | 841 | 1.2 |
| 3.00 | 5 | 832 | 1.1 |
Table 4 Coagulant dosage and effluent index
It can be seen that the presence of suspended matter in electrophoresis wastewater has a significant impact on COD, and the workshop has also added pretreatment measures: coagulation and air flotation.
The principle of flotation is used to treat a large amount of suspended matter in the water. After forming a large floc, tiny bubbles are attached and float to the liquid surface. The scraper removes these impurities from the water surface, and the collected scum is then processed in the filter press.
The pretreated electrophoresis wastewater is then added to the comprehensive wastewater at a ratio of 1:3 for treatment, which reduces the impact on subsequent treatment.
Hydrogen peroxide wastewater pretreatment
Electrophoresis and phosphating water washing tanks have been used in automobile paint for a long time. The electrophoresis paint contains eutrophic acid, and the suitable temperature may breed bacteria, block the anode system pipeline, and cause the anode liquid to overflow into the electrophoresis tank.
Excessive bacterial reproduction in the electrophoresis tank will lead to incomplete paint film and blistering of the car body, affecting the following process: reprocessing of unqualified products consumes a lot of energy and dramatically impacts the entire painting process.
Therefore, it is necessary to sterilize and disinfect the water washing tank regularly. The conventional method is to use 6% hydrogen peroxide for sterilization. After several hours of pipeline circulation soaking, rinse with clean water and discharge the wastewater into the collection pool. Hydrogen peroxide is unstable and easily decomposed into water and oxygen.
Because hydrogen peroxide continuously releases oxygen during decomposition, especially when the pH reaches about 10.5, the decomposition speed is much faster under alkaline conditions. The oxygen bubbles formed in the water adhere to the sludge, causing the precipitated initial flocs to float and affecting the effluent quality. In severe cases, even large flocs float to the surface of the pre-separation tank, and the precipitated initially and separated sludge overflows to the Lamella Clarifier tank and other subsequent treatment units.
For this type of wastewater, it is necessary to collect and pre-treat separately, add liquid alkali to adjust the pH to about 10, stir, and let it stand for 4~5 days; the hydrogen peroxide in it can be completely decomposed and disappear, and then the wastewater is treated together with the usual sewage, and the floating mud phenomenon will no longer appear in the pre-separation tank.
Phosphate wastewater homogenization treatment
In the phosphating process of the automobile coating process, phosphating liquid is used, and the phosphorus content is very high, sometimes up to 1 g/L. In the normal overflow of the phosphating tank liquid and the discharge of the spray water and washing water, a large amount of phosphate ions are contained, and there is a lot of orthophosphate in the phosphating wastewater.
There are many processes for treating total phosphorus in wastewater, but the commonly used ones are chemical precipitation, ion exchange and biological phosphorus removal. However, for the treatment of phosphating wastewater, chemical precipitation is generally used due to the large construction investment of the ion exchange method, strict requirements on equipment materials, and complex operation and management.
Because of its cost saving and simple operation, it is currently one of the most commonly used methods for treating phosphating wastewater, but the sludge output is large. Chemical phosphorus removal refers to the addition of chemical agents, such as PAC, ferric chloride solution and lime milk, which react with wastewater to form flocs and remove total phosphorus into the sludge.
At present, the chemical precipitation method is used to treat phosphating wastewater in workshops, and lime milk is added to form calcium salt precipitation. Calcium salt is used to treat phosphorus-containing wastewater. Phosphate ions react with calcium ions. The total phosphorus (P) concentration of wastewater entering the buffer tank from the phosphating wastewater pool is generally 100~150 mg/L. After sufficient adjustment and mixing, the water quality and water volume of the wastewater are relatively stable. The amount of lime milk added and the effluent indicators are shown in Table 5.
| Lime milk dosage/(L·t-1) | TP/(mg·L-1) | TNi/(mg·L-1) | COD/(mg·L-1) | Sludge production/(kg·t-1 water) |
| 0.50 | 20.2 | 0.45 | 689 | 3.64 |
| 1.00 | 7.5 | 0.29 | 573 | 3.89 |
| 1.25 | 6.4 | 0.21 | 521 | 4.73 |
| 1.50 | 3.2 | 0.16 | 432 | 4.12 |
| 1.75 | 2.7 | 0.11 | 367 | 5.31 |
| 2.00 | 2.3 | 0.08 | 359 | 6.01 |
| 2.50 | 1.5 | 0.05 | 351 | 6.57 |
Table 5 Lime milk dosage and water output index
There are many process paths for the source of coating wastewater, which produces wastewater of various types, concentrations, and water volumes. The workshop collects wastewater according to its nature and discharge volume and adopts targeted treatment methods for pretreatment, which can save reagents and treatment time.
If high-concentration polluted wastewater is treated centrally, the total amount of pollution will be increased, the difficulty of treatment will be increased, the treatment cost will increase, the adjustment variables of the entire sewage treatment will also increase, and the uncertainty factors will increase. The classified collection has a relatively straightforward system management record of the composition, water volume, and pollution concentration of each type of wastewater. This is conducive to accurately grasping the water consumed by the coating process in each section. Abnormal discharge of wastewater can also be reflected in time through classified collection and feedback, and countermeasures can be taken for changes in wastewater caused by changes in the coating production process. Pretreatment of some high-concentration wastewater can greatly reduce the concentration of pollutants, especially some impurities that are difficult to degrade and remove, which reduces the subsequent treatment load and helps to improve the overall treatment efficiency.
Uniform mixing of water quality and quantity before wastewater treatment can ensure the reagent's dosage stability, reduce the impact of fluctuations in water quality and quantity, and make the water treatment operation more stable and efficient.
