With the rapid development of the domestic automobile industry, high-end furniture industry, IT industry, etc., the number of supporting plastic electroplating enterprises has increased rapidly, and the plastic electroplating wastewater generated has also increased. Because the production process of plastic electroplating is more complicated than that of metal electroplating, the pollutants in its wastewater are also more diverse, and the treatment process and methods are different from those of metal electroplating wastewater. In addition, due to the promotion of the national circular economy policy, energy conservation and emission reduction have become important tasks for governments at all levels and enterprises. Many provinces and cities across the country, such as Shanghai, Guangdong, and Jiangsu, have made regulations on the wastewater reuse rate of electroplating enterprises, so the reuse of electroplating wastewater has become inevitable.
When the electroplating copper washing water is not reused online, it is directly discharged into the mixed sewage for treatment; when it is reused online, the RO concentrated water of its reuse system is also discharged into the mixed sewage for treatment. When the plastic electroplating nickel washing water is not reused online, it is discharged into the chemical nickel sewage for treatment; when it is reused online, the RO concentrated water of its reuse system is also discharged into the chemical nickel sewage for treatment. Therefore, the treatment system of plastic electroplating wastewater is similar to that of metal electroplating wastewater, and can be divided into nickel-containing wastewater treatment system, chromium-containing wastewater treatment system, and mixed wastewater treatment system.
1. Treatment of nickel-containing wastewater
Unlike general nickel-containing wastewater, nickel-containing wastewater from plastic electroplating contains chemical nickel wastewater. There are a lot of hypophosphites and organic complexes in the wastewater. Only simple adjustment of pH value and coagulation and sedimentation treatment cannot remove the nickel ions in the form of complexes in the water. At the same time, the removal of hypophosphites also needs to be targeted. Generally, it is necessary to adjust the pH of the sewage and add oxidants to convert hypophosphites and phosphites in the sewage into phosphates, and destroy the complexes in the sewage to free nickel ions. Then, calcium salts, coagulants, and flocculants are added, and the phosphates and nickel ions are removed after the reaction to form sediments. It is worth noting that because the nickel-containing wastewater treatment system needs to treat the RO concentrated water of the electroplating nickel washing water reuse system, it is difficult for the COD in the water to reach below 100 mg/L after only one coagulation and sedimentation process, so the general practice is to discharge it into mixed sewage for further treatment after treatment.
2. Treatment of chromium-containing wastewater
Chromium-containing wastewater uses the usual chemical reduction method to reduce Cr6+ to less toxic Cr3+, and adjusts the pH value to form Cr(OH)3, and removes chromium after sedimentation. The same as the nickel-containing wastewater treatment system, when the chromium-containing wastewater is reused online, the RO concentrated water generated by it is difficult to reach below 100 mg/L after one coagulation and sedimentation, so its effluent should also be discharged into mixed sewage for continued treatment.
3. Treatment of mixed wastewater
The types of pollutants in plastic electroplating mixed wastewater with an online recycling system are much more complicated than those without an online recycling system, because the RO concentrated water of the online recycling system must eventually be fed into the mixed wastewater for treatment, so its treatment is more complicated. The common treatment process is: chemical oxidation -> pre-biochemical coagulation and sedimentation -> hydrolysis and acidification -> contact oxidation -> post-biochemical coagulation and sedimentation -> discharge.
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