Treating Wastewater in the Photovoltaic Industry (Part One)

Treating Wastewater in the Photovoltaic Industry (Part One)


Abstract: Aiming at the development of wastewater treatment technology in the photovoltaic industry, different photovoltaic wastewater treatment processes are compared and summarized to provide a reference for wastewater treatment in the photovoltaic industry in this article. The photovoltaic industry is an important industry for the conversion and utilization of solar energy. Although solar energy is a clean energy source, the production line of the crystalline silicon solar panel in the mainstream industry requires a lot of water and produces a lot of waste water. This kind of waste water has poor biochemical properties, a great difference in pH due to different processes, and high fluoride ion content. Photovoltaic enterprises usually deal with the wastewater in the factory, and then discharge it to the municipal pipes. The wastewater treatments are also different due to the different processes of crystalline silicon solar panels, and they can be roughly divided into two categories: treatment by water quality or flow and mixed treatment.
 
PV is short for solar photovoltaic power generation systems. The photovoltaic industry is related to the production, research and development, integration and operation of monocrystalline silicon, polycrystalline silicon solar cells, modules and related products that convert light energy into electrical energy. Among these industries, crystalline silicon modules have a market share of 90%, which is the mainstream industry in the photovoltaic industry. Solar energy is environmentally friendly, sustainable, and can alleviate the shortage of energy in the world, making the photovoltaic industry flourish globally. According to forecasts, the cumulative global photovoltaic installed capacity is expected to reach 1721 GW by 2030, and will further increase to 4670 GW by 2050. In addition, China has the largest photovoltaic installation market. As of 2020, China has maintained the number one in the world for 6 years. The output of wastewater in the manufacturing process cannot be underestimated. Therefore, the wastewater from the photovoltaic industry must be treated economically and effectively to ensure the sustainable development of the green industry.
 
1. The characteristics of the photovoltaic industry and its wastewater
Although solar energy is a green energy source, as a mainstream industry, the manufacturing process of crystalline silicon generates a lot of wastewater. For solar panels, the commonly seen products are monocrystalline silicon and polycrystalline silicon solar panels. However, for both solar panels, strong oxidizing solutions such as chromic acid, nitric acid, hydrofluoric acid and sulfuric acid are used to clean, texturize and etch silicon wafers to ensure that crystalline silicon can absorb solar energy to a great extent; isopropanol, ethanol and heavy metals are added as additives in the manufacturing process. Therefore, the wastewater has the main characteristics of having low pH, high nitrate nitrogen content, high fluorine ion content, low biodegradability, and heavy metals. In recent years, liquid ammonia, hydrogen peroxide and other raw materials are added to improve the production technology, and the quality of wastewater needs to meet the requirements for Discharge Standards for Pollutants of the Battery Industry (GB30484-2013).
 
2. Commonly seen processing technologies
2.1 Segmented wastewater treatment
The manufacturing processes of crystalline silicon solar panels include cleaning, texturing, cutting, grinding, and etching, and the main pollutants of wastewater in each section are different. The water's quality and quantity vary greatly. The specific parameters of the water quality are shown in Table 1. According to the concepts of qualitative collection and segmented processing, each processing unit can treat the main pollutants stably and efficiently, but the processing procedures and structures are complicated. Some wastewater treatment is a mixed treatment of certain stages of wastewater, and the water quality is shown in Table 2. The mixed treatment process pipeline will be simpler. The acid and alkali can be neutralized to treat the wastewater due to the different acidity and alkalinity of the wastewater in different sections. However, the structure of the tank body needs to be large. The hydraulic retention time is the same to meet the treatment effect, and the treatment efficiency is lower than treatment by different water quality.
 
Generally, chemical precipitation plus coagulation are adopted for wastewater containing fluorine. For example, when designing wastewater treatment engineering of polycrystalline silicon wafer, Xingming Tao and others adopted a two-stage reaction precipitation process for wastewater containing fluorine. Ca(OH)2 and PAM were added for the first stage precipitation. CaCl2 and PAM were added for the second stage. The difference in treating wastewater containing fluoride was that PAM was added in the process when CaF2 was produced. The cutting waste water was treated by coagulation and sedimentation of PAC and PAM. In the treatment; each stage of the wastewater was provided with a regulating tank, and the process and pipeline layout were slightly complicated. Lixia Feng and others studied the wastewater from the production of high-pressure silicon stacks and silicon materials in Tianjin. The wastewater containing fluorine was treated with a coagulation sedimentation process. CaF2 was generated by adding CaCl2, and the subsequent addition of PAC and PAM accelerated the precipitation of CaF2. Since pH was one of the key influencing factors for removing fluoride, the optimal pH for coagulant dosage was from 8.5 to 9.5 through orthogonal experiment. Xuefeng Xiao and others pointed out that different calcium sources had an impact on the efficiency of removing fluoride in the study of calcium precipitation coagulation treatment of wastewater with high fluorine generated by the production of solar cells. Using Ca(OH)2 and CaCl2 as a composite calcium source could well remove fluoride ions. The effect was good, but the supernatant would be turbid. It was concluded that the higher the Ca2+ content was added, the better the F removal effect became. The lower the F content was, the more difficult it was for the fluoride to be removed. Moreover, the three-stage series-intensified coagulation process could make F stably drop below 10mg/L. Weihui Chen and others used a step-by-step addition of Ca(OH)2 and CaCl2 to treat wastewater containing fluorine in a solar power company in Wuxi. Co-solubilization occurred for CaF2 and Ca(OH)2 after the reaction. Even if an excessive amount of Ca(OH)2 was added, the effect of removing the fluoride was not good. Qianqian Liu and others used the processes such as adjustment, adsorption, filtration, the first stage reverse osmosis, adjustment and the second stage reverse osmosis to treat wastewater containing fluorine in the photovoltaic industry to meet the requirements of Urban Sewage Recycling Industrial Water Quality (GB/T19923-2005).
 
Organic wastewater usually enters the biological treatment unit. Although photovoltaic wastewater has a high COD content, its biochemical performance is poor. It needs to be treated to improve biodegradability in the early stage. For example, Shu Chen and others treated wastewater from a production line of a monocrystalline silicon chip in Liaoning. Processes such as grilles, coagulation sedimentation, hydrolytic acidification, contact oxidation and MBR membrane were adopted. After treatment, 50% of the wastewater reached the municipal pipe standard and was discharged to the pipe. The other 50% of the wastewater could be recycled through ozone disinfection and activated carbon adsorption. Among them, hydrolysis acidification was set to improve biodegradability. Cleaning waste liquid was also a highly organic waste liquid. The processes used by Xingming Tao were adjustment, coagulation sedimentation, anaerobism, facultative oxygen and contact oxidation, and the treated wastewater met the requirements for the grade 3 emission standard of Discharge Standards for Integrated Wastewater (GB 8978-1996). The processes adopted by Mingjiang Peng and other were adjustment, coagulation and sedimentation, intermediate pool, EGSB, ABR and contact oxidation. Because it is difficult to degrade high-molecular organics, anaerobic treatment was adopted for the first two parts of the biological treatment of this process. The stability of two-stage anaerobic was better than that of single-stage anaerobic.