Automotive Coating Wastewater Treatment

Huicong Surface Treatment Network: Coating processes are extensively used in automotive surface treatment, and wastewater is generated in virtually every stage of the production process [1]. As a result, the treatment of automotive coating wastewater has become a major challenge in today’s wastewater treatment projects, urgently requiring solutions. Through years of exploration and engineering practice, it has been found that adopting physicochemical plus biochemical treatment for automotive coating wastewater is both economically viable and effective. However, in practical applications, certain issues still remain, necessitating optimization and improvement of this process to make automotive coating wastewater treatment more stable and efficient.

1. Sources and Characteristics of Automotive Coating Wastewater

The wastewater generated in the coating process mainly includes pretreatment wastewater from processes such as degreasing, acid pickling, and phosphating surface conditioning, electrophoretic coating wastewater, and spray wastewater from applying primer, mid-coat, and topcoat [2]. The composition of each stream of wastewater is complex, with varying concentrations, making treatment particularly challenging.

With the exception of a portion of the wash water that continuously overflows from the sump, wastewater and waste liquids generated in each process are mostly discharged intermittently. After these streams of wastewater are mixed together, they form highly turbid coating wastewater. The volume and quality of this wastewater vary significantly throughout the day and exhibit no discernible pattern. The wastewater contains a complex mixture of pollutants, including numerous toxic substances at high concentrations, and has poor biodegradability. According to years of monitoring, the overall water quality characteristics are as follows: CODcr concentration ranges from 1000 to 2500 mg/L, BOD5 concentration ranges from 100 to 250 mg/L, SS concentration ranges from 400 to 600 mg/L, petroleum hydrocarbon concentration ranges from 30 to 85 mg/L, phosphate concentration ranges from 25 to 50 mg/L, and pH...

7.0–8.5, Zn²⁺ concentration 5.0–20 mg/L.

2. Research on the treatment process

2.1 Simple Physicochemical Method

Due to the poor biodegradability of automotive coating wastewater, a typical physicochemical treatment process generally consists of the following steps: equalization basin—coagulation and sedimentation or air flotation—sand filtration—activated carbon filtration. In some processes, the wastewater from each stage is separated and subjected to individual pretreatment by adding chemicals for reaction (for example, oil-containing wastewater is treated with chemicals to break emulsions) before undergoing coagulation and sedimentation or air flotation. In theory, this process is feasible for treating coating wastewater when appropriate coagulants and flocculants are selected. However, after simple physicochemical treatment, the effluent quality is unstable. After coagulation and sedimentation or air flotation, the COD removal rate typically ranges from 30% to 60%, with a maximum of 80%. As a result, the effluent COD concentration remains around 450 mg/L, and most of this COD consists of water-soluble organic compounds. The removal of these water-soluble organics relies primarily on activated carbon adsorption. Increasing the load on the activated carbon filter rapidly leads to its exhaustion, thereby causing the effluent to fail to meet the required standards. Moreover, the process flow is lengthy, operation and maintenance are complex, and operating costs are high.

2.2 Combined physicochemical and biochemical treatment method

Currently, one of the most promising approaches for treating automotive painting wastewater is the physicochemical-plus-biological method. The core principle of this process is to use physicochemical treatment as a pretreatment step, followed by biological treatment, thereby ensuring that the wastewater consistently meets discharge standards.

(1) Physicochemical Pretreatment

Since automotive coating wastewater contains large amounts of substances—such as phosphates—that cannot be completely removed by biochemical methods or are difficult to remove, it is essential to rely on physicochemical processes for their removal. In practical engineering applications, lime is often used; lime milk is employed to maintain the pH of the wastewater above 11.5, enabling the formation of hydroxyapatite and zinc hydroxide precipitates from phosphate ions and zinc ions, thereby reducing the phosphate concentration in the wastewater below 5.0 mg/L. At the same time, Ca²⁺ facilitates the destabilization and coagulation of emulsified oils and polymeric resins, creating favorable conditions for the coagulation reaction.

(2) Biochemical treatment

After pretreatment by physicochemical methods, the wastewater quality is improved to some extent; however, it can only stably meet discharge standards after undergoing biochemical treatment. Since the major pollutants in the painting workshop wastewater have poor biodegradability (BOD/COD = 0.1), enhancing the biodegradability of the raw wastewater is the primary prerequisite for its biochemical treatment. Secondly, given the imbalanced nutrient composition in industrial wastewater, it is necessary to add nutrient sources to improve the wastewater’s biodegradability. On the other hand, prior to biochemical treatment, the wastewater is first subjected to hydrolysis and acidification—i.e., the anaerobic process is controlled at the hydrolysis-acidification stage. Through the action of hydrolysis-acidification bacteria, difficult-to-degrade synthetic organic compounds such as epoxy resins, cyclic organic compounds like ethers, and aromatic organic compounds are broken down into smaller molecular organic substances, thereby significantly improving the wastewater’s biodegradability.

After undergoing hydrolysis and acidification treatment, the wastewater is further processed using an aerobic process. The aerobic biochemical stage is the core component of the entire wastewater treatment system. Under aerobic conditions, biodegradable pollutants in the wastewater are converted—partly into microbial cells and partly into CO2 and H2O—through the action of aerobic microorganisms, thereby achieving thorough removal. Excess microbial biomass is removed from the system via sludge discharge, thus purifying the water quality.

Among the aerobic processes commonly used in engineering practice are the SBR process and the contact oxidation process. Due to the significant variability in both the water quality and quantity of automotive coating wastewater, the contact oxidation process is difficult to operate stably, resulting in substantial fluctuations in effluent quality. Therefore, supplementary treatment steps such as microflocculation filtration or activated carbon adsorption are required to ensure that the effluent consistently meets discharge standards. In contrast, the SBR process features a cyclic operation encompassing inflow, aeration and reaction, quiescent settling, supernatant removal, and idle periods—all integrated into a single unit. This process boasts stable operational performance, high resistance to fluctuations in water volume and organic load, flexible operation, simple construction, and convenient operation and maintenance[4]. Consequently, the SBR process is widely applied in the treatment of automotive coating wastewater.

2.3 Process Flow

Taking the wastewater treatment for painting at an automobile manufacturing company in Hunan as an example, the designed treatment capacity is Q = 300 m³/d. The water quality is as described previously. The process flow is as follows:

Due to the irregular tank-reversal process in the coating pretreatment stage, the waste liquid from tank reversal is discharged intermittently in large volumes and at very high concentrations, necessitating separate treatment based on waste quality. The concentrated waste liquid from tank reversal is collected in a concentrated waste liquid tank, while other wastewater with lower concentrations is directed to an equalization basin. Then, a pump periodically and quantitatively transfers the concentrated waste liquid into the equalization basin, where it is thoroughly mixed with the other wastewater. In the coagulation reaction tank, lime milk and PAM are added and thoroughly mixed; after sufficient reaction, most of the phosphate, heavy metals, and suspended solids are removed. Subsequently, after sedimentation and clarification, hydrochloric acid is added to adjust the pH of the wastewater. After physicochemical treatment, the effluent undergoes hydrolysis and acidification before entering the SBR reactor, where aerobic biochemical reactions take place. During these reactions, organic pollutants in the wastewater are aerobically degraded, thereby purifying the wastewater and enabling it to meet the national Class I emission standards.

3. Improvement of the process

Through the design and actual operation of several automotive coating wastewater treatment plants, it has been found that adopting a physicochemical-plus-biological treatment process for coating wastewater is economically feasible and can achieve the expected treatment results. However, some issues still remain, necessitating optimization and improvement of this process.

3.1 Uniform Water Quality and Quantity

Since automotive painting wastewater is mostly discharged intermittently, with large instantaneous discharge volumes and high concentrations, it must be thoroughly mixed in the equalization basin to minimize the impact on subsequent treatment processes. When designing an equalization basin, it is essential to ensure that the wastewater remains in the basin long enough to achieve uniform mixing. Typically, the effective volume of the equalization basin should account for at least 40% of the designed flow rate. During operation, special attention must be paid to leaving sufficient safety volume within the basin to dilute the highly concentrated wastewater pumped from the waste liquid tank, thereby preventing significant fluctuations in water quality that could destabilize the entire system.

3.2 Control of Chemical Phosphorus Removal

The phosphate concentration in automotive painting wastewater is relatively high, necessitating the adoption of physicochemical methods for phosphorus removal. During operation, an excess amount of lime milk is added to adjust the wastewater pH to above 11.5, which not only removes heavy metal ions but also serves as an inexpensive and highly effective phosphorus-removal agent. Based on actual operational experience, when lime is used as a coagulant and PAM as a flocculant, the phosphate removal rate can reach approximately 99%, with the effluent phosphate concentration dropping below 0.5 mg/L. However, such highly efficient chemical phosphorus removal can result in excessively low phosphate levels in the wastewater, thereby affecting the subsequent biochemical reactions. Therefore, it is essential to carefully control the dosage of lime milk to ensure that the phosphate concentration in the effluent remains within the range of 2.0 to 3.0 mg/L—this not only meets the requirements of biochemical reactions but also guarantees that the final effluent phosphate levels consistently comply with regulatory standards.

3.3 Supplementing Wastewater with Nutrients

Since automotive coating wastewater lacks various nutrients required by microorganisms, it is necessary to consider supplementing the wastewater with nutrients.