Treating Wastewater in the Photovoltaic Industry (Part Two)

Treating Wastewater in the Photovoltaic Industry (Part Two)


Table 1 Water quality parameters of wastewater of crystalline silicon solar panels by segmented processes
Sources of wastewater Water quality
pH COD BOD SS Fluoride
Wastewater containing fluoride 2 to 6 750 340 Less than 100 350
Organic waste water 6 to 9 850 360 Less than 100 Less than 10
Granular wastewater 6 to 9 1500 680 Less than 1500 Less than 10
Alkaline washing waste liquid 13 to 14 13000 to 15000 340 3000 to 4000  
Pickling waste liquid 1 to 2 2 1000 to 25000 800 5000 to 6000  
Cutting waste water 8.5 3500 6800 1000  
Cutting fluid recycling sand washing water 4 to 6 34000 200 3000 Less than 100
Cutting fluid recycling pickling water 1 to 5 850 2500 300  
Cutting fluid recycling alkaline washing water 9 to 12 9000 400 2500  
Washing water 6 to 9 900 350 400  
Slicing production line wastewater 6 to 7 3000 800 1800  
 
2.2 Mixed wastewater treatment
Jie Sun and others put the waste water from each section of the monocrystalline silicon solar cell to a comprehensive waste water adjustment tank. The advantages were that the acid and alkali waste liquid could be mixed to achieve the purpose of using waste to treat waste. After that, three-stage coagulation sedimentation was selected. The first two stages were to remove fluoride ions. The last stage was to prevent sludge calcification in the subsequent biological treatment. Na2CO3 was used as the precipitant for calcium removal, and the two-stage AO process at the end was used to remove nitrates and organics. The low CIN was supplemented by carbon sources in the anaerobic stage of the two-stage process; sodium acetate was added to the process, and the final effluent reached the indirect emission standard of the Emission Standard of Battery Industry Pollutants (GB30484-2013). Shuishui Zhang researched the treatment of wastewater produced by the manufacturing of a polysilicon cell in Zhangjiagang City, Jiangsu Province. The main process was to mix and treat the wastewater from the cleaning and texturing and etching processes. The main processes were adjustment → two-stage precipitation → sand filtration → carbon filtration → ion exchange → two-stage reverse osmosis → evaporation and crystallization. The final effluent reached the reuse standard, and the cost of wastewater treatment per ton is 17.52RMB. Feng Zhu used the processes: adjusting → Fenton catalytic oxidation→ hydrolytic acidification → SBR effluent COD meeting the GB8978-1996 grade 1.
 
Table 2 Water quality parameters of overall wastewater produced by crystalline silicon solar panels
Sources of wastewater Water quality
COD/(mg/L) BOD5/(mg/L) SS/(mg/L) pH F/(mg/L) TDS
Comprehensive wastewater 3200 800 800 6.2 to 6.8    
Less than and equal to 30     2 to 5 200 to 800 Less than 5000
Less than and equal to 3000 1500 Less than and equal to 800 5 to 7    
2500 to  3000 1000 to 1500 1000 to 1500 6 to 9 20 to 30  
1000   300 1 to 2 1200  
 
3. Conclusion
Wastewater treatment generated by the rapid development of the photovoltaic industry is also urgent. Throughout the past few years, there have been two overall treatments for photovoltaic wastewater, which are mixed treatment and classified and qualitative treatment.
 
Adjustment water quality and coagulation and sedimentation were used in the pre-treatment, and the effect of coagulation and sedimentation was ensured through pH adjustment. The biodegradability of the organic wastewater in the separation or the mixed treatment of wastewater was very poor. In order to improve the microbial impact resistance, anaerobic treatment was generally selected. In order to stabilize the treatment effect, it usually requires two-stage anaerobic treatment in series. You can choose different reactors and some processes equipped with hydrolytic acidification or Fenton oxidation to improve biodegradability. Moreover, the effluent generally does not meet the standard due to the characteristics of anaerobic treatment, and an aerobic biological treatment must be added later. In order to make F- discharge meet the standard, Ca2+ is usually excessive. After defluorination, wastewater with high Ca2+ content will enter the biological treatment unit. Although Ca2+ is necessary for the growth of microorganisms, excessive Ca2+ affects the normal combination of other heavy metals and some enzymes, affecting the normal metabolism of microorganisms. In addition, the process of microbial respiration produces CO2 and Ca2+, forming CaCO3 wrapped on the surface of sludge and hindering the circulation of microorganisms and wastewater, that is, sludge calcification. In order to ensure the normal progress of biochemical treatment, the Ca2+ concentration of the reactor should be lower than 600mg/L. When it is too high, Na2CO3 is usually added to remove calcium.
 
The difference between the two treatments is that in order to meet the emission standards in the mixed treatment process, many primary unit treatments in the process cannot reach the ideal state, so three-stage coagulation and two-stage reverse osmosis are common in the process. Moreover, the difference in acidity and alkalinity of each section of wastewater can achieve the purpose of treating waste by waste. The so-called classification or treatment by different quality generally refers to wastewater containing fluorine wastewater and organic wastewater. The so-called classification or treatment by different quality generally refers to wastewater containing fluorine wastewater and organic wastewater. The common one is the treatment and reuse of wastewater containing fluorine. Wastewater containing fluorine is characterized by low pH, high fluoride ions content and low other composition.
 
For the removal of fluoride ions, the composite precipitation agent of CaO2 and CaCl2 is usually selected. Although the price of CaO is low, the precipitation of CaF is poor. PAC and PAM need to be used for strengthening precipitation in the later stage. Because the hydrolysis of PAC is affected by pH, the dosage and coagulation effect are the best under neutral conditions. However, if CaO is only used to remove fluoride, the pH will reach alkalinity. Affecting by the uniformity and reaction degree of the agent in the wastewater, usually, the actual dosage of the agent is 1.1 to 1.3 times of the theoretical dosage, and the pH value will be higher, which is not only not economical, but also has poor precipitation effect. Therefore, the dosage of CaO is limited by pH, and CaCl2 is used to remove the remaining F-. A lot of KOH is used in the workshop due to the improvement of technology in the photovoltaic industry in recent years, making the pH increased for wastewater and further limiting the adding of CaO. In order to meet the water standard, the amount of CaCl2 will inevitably increase and the treatment cost will also increase. Subsequent addition of reverse osmosis process, ion exchange, and membrane treatment process can make effluent water quality reach the reuse standard, but the overall cost is high.
 
The two treatments have their own advantages. A single method cannot meet the treatment requirements economically and effectively. In addition, with the good development of the photovoltaic industry, the renewal of processes, the volume and quality of wastewater produced have also changed, and the national standards for industrial wastewater treatment are more strict. The flexible combination can be adopted according to manufacturing processes of the actual photovoltaic industry and quality and quantity of wastewater, which can not only meet the treatment requirements, but also be more adaptable.