Complex in composition, industrial wastewater often contains heavy metals, which can exceed safe levels in industries such as mining, mechanical manufacturing, chemical processing, electronics, and instrumentation. How are these heavy metals typically handled, and what are the methods used?
Heavy metals in wastewater are generally indestructible; they can only be transferred in their location and transformed into different physical and chemical forms. The treatment method involves first reforming the production process to either use or minimize the use of highly toxic heavy metals.
On-site treatment at production locations (such as not discharged from the production workshop) commonly employs methods like chemical precipitation, ion exchange, electrolytic recovery, and membrane technology for processing. The treated water, with heavy metals below the discharge standards, can be discharged or reused.
Chemical Precipitation Method
By adding heavy metal trapping agents, the removal rate is enhanced. Currently, most electroplating enterprises in China adopt various electroplating treatment processes, yet the wastewater still fails to meet standards. In such cases, the heavy metal trapping agent can be directly added to the existing coagulation tank in the process.
Ion Exchange Method
Ion exchange is primarily used for water softening and deionization, filtering raw water through ion exchange resins; the ions in the water will exchange with ions fixed on the resin.
After ion exchange, membrane filtration is conducted. Then, a heavy trap reagent can be added to the chemical dosing unit for efficient flocculation and sedimentation. Subsequently, the water is filtered and disinfected, achieving the standard for water treatment.
Electrolytic Recycling Method
Electrolytic recycling primarily utilizes the electrochemical properties of metals. During the process, pollutants undergo oxidation and reduction reactions at the anode and cathode, respectively, eventually transforming into non-toxic and harmless chemical substances or forming insoluble precipitates, thereby achieving the removal of heavy metals.
4 Membrane Techniques
Electrodialysis and reverse osmosis are the primary methods. The characteristic of electrodialysis is that the concentration ratio is limited; it requires multi-stage electrodialysis treatment to concentrate the useful substances in wastewater to a reusable level.
Reverse osmosis is used for treating electroplating wastewater, such as nickel, copper, zinc, cadmium plating rinses.
Therefore, heavy metal wastewater can be concentrated and reused through reverse osmosis treatment, and the reclaimed water (product water) can also be reused when its quality is good.
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