Engineering Application of Ozone Catalytic Oxidation in The Treatment of Monocrystalline Silicon Cutting Fluid Wastewater

Engineering Application of Ozone Catalytic Oxidation in the Treatment of Wastewater from Monocrystalline Silicon Cutting Fluid

Wastewater from monocrystalline silicon cutting fluid is characterized by high COD and poor biodegradability. Efficient treatment of such wastewater is an urgent need for pollution reduction, stable wastewater discharge compliance, and water environmental protection. This paper introduces the engineering application of adding an ozone catalytic oxidation pretreatment process before the biochemical process section in a sewage treatment plant. By optimizing the ozone dosage ratio, stable discharge compliance was achieved.

The cutting process of producing monocrystalline silicon in the photovoltaic industry generates a large amount of monocrystalline silicon cutting fluid wastewater. This type of wastewater is rich in high-molecular polymer polyethylene glycol, resulting in a COD as high as 10 - 20g/L. It also contains high-concentration suspended solids such as silicon powder and silicon carbide, fluoride ions, acids and bases, as well as pollutants like detergents, cutting fluids, and surfactants. It has the characteristics of high COD and SS content, poor biodegradability, and difficult-to-degrade pollutants. Currently, it is proposed to treat this wastewater within the enterprise through processes such as air flotation, biochemical treatment, and biological aerated filter, until it meets the discharge standards, and then mix it with other wastewater and send it to the sewage treatment plant for biochemical treatment. However, this approach poses a risk of water quality shock to the downstream sewage treatment plant, affecting its stable operation.

There are numerous technologies for treating refractory wastewater both at home and abroad, such as activated carbon adsorption, hydrolysis acidification, biological contact oxidation, micro-electrolysis, and Fenton oxidation. However, all these methods have certain limitations. The activated carbon adsorption method has complex engineering measures and problems of adsorption saturation and selectivity; the micro-electrolysis method has high operating costs and is difficult to manage; the hydrolysis acidification method requires high energy consumption to maintain biological activity and water circulation; the biological contact oxidation method has a long operation time and low treatment efficiency; the Fenton oxidation method has a long reaction time and leaves a large amount of metal iron ions in the wastewater, and the iron sludge produced is prone to secondary pollution of the water environment. Therefore, it is urgent to find a more efficient and environmentally friendly treatment method. The advantages of oxygen catalytic oxidation

Ozone catalytic oxidation technology can induce the generation of a large amount of ·OH with high redox potential, thereby effectively removing and even mineralizing organic substances in wastewater. This technology has the advantages of fast reaction speed and wide application range. Generally, no sludge is produced during the treatment process, and no post-treatment is required. The O3 in the treated wastewater is easy to decompose and does not cause secondary pollution. Compared with other technologies for treating refractory wastewater, ozone catalytic oxidation technology has obvious advantages in treating monocrystalline silicon cutting fluid wastewater.

When treating monocrystalline silicon cutting fluid wastewater, ozone catalytic oxidation technology can utilize the lanthanide rare earth metal catalyst loaded on silicon carbide in the tank to catalyze the generation of ·OH. Through its strong oxidation effect, the COD in the monocrystalline silicon cutting fluid wastewater can be further reduced. At the same time, this technology can also be combined with other processes. For example, adding an ozone catalytic oxidation treatment process before the biochemical process in a sewage treatment plant can effectively improve the quality of the influent to the biochemical process, reduce the subsequent treatment load, and provide a strong guarantee for the stable and standard discharge of sewage. Process renovation design .

This project, on the basis of the existing treatment of wastewater from monocrystalline silicon production enterprises, adds an ozone catalytic oxidation treatment process section in the downstream sewage treatment plant. The treatment process is as follows: The monocrystalline silicon cutting fluid wastewater, after being treated by the production enterprise, is conveyed through a dedicated pipeline to the sewage treatment plant. It is then lifted by a lift pump to the regulating tank where NaOH is added to adjust the pH, to balance the water quality and quantity of the monocrystalline silicon cutting fluid wastewater. Subsequently, it is conveyed to the high-efficiency sedimentation tank where a flocculant is added to further remove SS from the monocrystalline silicon cutting fluid wastewater. The effluent from the high-efficiency sedimentation tank is further treated by a precision filtration unit to remove SS from the water before entering the ozone catalytic oxidation tank.

The additional process units include a regulating tank, high-efficiency sedimentation tank, precision filtration unit and ozone catalytic oxidation tank. The regulating tank is equipped with two submersible sewage pumps with a designed water flow of 250 m3/h and a head of 10 m. It is also equipped with an air agitation system to ensure uniform mixing of the wastewater. Chemicals are added to adjust the pH of the liquid in the tank to 7 - 9 based on the pH value inside the tank. The high-efficiency sedimentation tank is a semi-underground reinforced concrete tank with a high-density sedimentation tank. It is equipped with paddle-type agitators, submersible sewage pumps, lift agitators, center-driven sludge scrapers and return pumps, etc., to reduce the SS content in the wastewater. The precision filtration unit adopts an above-ground frame structure, with two precision filters inside, having a filtration accuracy of 10 μm. A buffer tank is set below the filtration facilities, and two buffer tank lift pumps are installed in the tank to transport the wastewater to the ozone catalytic oxidation tank. Capacity optimization

The ozone dosage per unit mass of COD (denoted as the ozone dosage ratio) of the project was optimized, and the water quality changes during the optimization experiment period were investigated. During the first to fifth days of system operation, the average influent COD of the monocrystalline silicon cutting fluid wastewater was 312 mg/L. The ozone dosage ratio gradually increased from 0.29 mg/mg to 0.80 mg/mg, and the O3 consumption per unit mass of COD removed (denoted as the ozone consumption ratio) also increased from 0.67 mg/mg to 1.33 mg/mg. Overall, the COD removal rate gradually increased, and the effluent COD decreased from 171 mg/L to 151 mg/L.

During the 6th to 12th day of the system operation, the average influent COD was 280 mg/L. At this time, the ozone dosage ratio fluctuated within the range of 0.98 - 1.39 mg/mg, with an average of 1.20 mg/mg. The average ozone consumption ratio also increased to 1.56 mg/mg. The average effluent COD from the ozone treatment unit was 83 mg/L, meeting the design standard for the effluent of the newly added process section. At this point, the average effluent COD of the wastewater treatment plant under management was 17 mg/L, meeting the discharge requirements of the A standard in the local standard DB 12/599 - 2015 of Tianjin. Subsequently, with the further increase in ozone dosage and the decrease in influent COD, the average ozone dosage ratio was 1.81 mg/mg, and the average effluent COD was 69 mg/L. However, the ozone consumption also significantly increased, with the average consumption ratio reaching 2.54 mg/mg. Moreover, the increase in ozone dosage would also cause high load and high energy consumption of the ozone tail gas destruction device. Therefore, it is more appropriate to control the ozone dosage ratio within the range of 0.98 - 1.39 mg/mg. Cost analysis.

The operating costs of the newly added ozone catalytic oxidation treatment process in this project include electricity fees, maintenance fees, labor costs, etc. The ozone dosage ratio is controlled at approximately 1.2 mg/mg. After the pretreatment system operates stably, the operating cost per ton of water treated by the ozone catalytic oxidation process is 14.5 yuan. Among them, labor costs account for 17%, electricity fees account for 56%, chemical agent fees account for 8%, maintenance fees account for 7%, sludge disposal fees account for 8%, and the costs for online equipment operation and maintenance and equipment inspection account for 4%.

Although the ozone catalytic oxidation treatment process section incurs certain operating costs, from an overall perspective, the addition of this process section effectively improves the water quality of the influent to the biochemical section, reduces the subsequent treatment load, and ensures stable and compliant effluent discharge. In the long run, it is of great significance for pollution reduction, stable effluent compliance, and water environment protection. Meanwhile, by optimizing measures such as the ozone dosage ratio, the operating costs can also be reduced to a certain extent and the treatment efficiency can be enhanced. Project Conclusion .

This project focuses on the wastewater from monocrystalline silicon production enterprises. On the basis of the existing treatment processes such as air flotation, biochemical treatment, and biological aerated filter, an ozone catalytic oxidation pretreatment process section is added. The operating conditions of the experiment are optimized by investigating the effect of ozone dosage ratio on the removal of COD in wastewater. The results show that when the average ozone dosage ratio is approximately 1.20 mg/mg, the average COD in the effluent from the ozone treatment unit is about 83 mg/L, and the final COD in the effluent from the downstream wastewater treatment plant is about 17 mg/L, achieving stable compliance with effluent water quality standards.

The application of ozone catalytic oxidation technology in the treatment of single crystal silicon cutting fluid wastewater provides an effective solution to the treatment of such wastewater. By rationally designing engineering renovation plans and optimizing the ozone dosage ratio, the advantages of ozone catalytic oxidation technology can be fully exploited to achieve stable and compliant wastewater discharge. This also offers reference and guidance for the treatment of other similar refractory wastewater. In the future, further in-depth research on ozone catalytic oxidation technology can be conducted to continuously optimize the treatment process, reduce operating costs, and improve treatment efficiency, making greater contributions to water environmental protection.