HIGH-TECH WASTEWATER TREATMENT SOLUTIONS IN THE PLATING INDUSTRY

1. Discharge sources of plating wastewater

1.1 Classification of plating technologies:

• Electroplating: the most widely used method today. Electroplating is an electrochemical process at the cathode. The metal surface acts as the cathode in an electrolytic cell powered by an external direct current, and the plating solution, which contains the metal ions to be plated, acts as the anode. These metal ions take part in the cathode reaction and are electrochemically reduced to metal, which deposits onto the surface being treated
• Electroless (chemical) plating: a method based on chemical reduction, in which metal ions are reduced to metal from their salt solution by reducing agents. The electrons needed for the reduction are supplied by the chemical reducing agents. For example, in nickel plating, sodium hypophosphite is used as the reducing agent
• Hot-dip plating: a process in which the material to be plated passes through a bath of metal (pure metal) that is melted at high temperature, so that the plating metal adheres to the surface of the material.

1.2 Basic plating industrial wastewater treatment process:

1.3 Sources of plating wastewater

• Wastewater from the plating process: The solution in the plating tanks can leak, spill or be carried out on plating racks and parts. After a period of operation, plating tanks must be cleaned, so dirt and sludge are discharged with the wastewater. As a result, the volume of wastewater is not large, but the pollutants are diverse and highly concentrated (Cr⁶⁺, Ni²⁺, CN^–…)
• Wastewater from surface cleaning of parts: Metal surfaces are usually coated with oil and grease from maintenance and mechanical polishing. To ensure the quality of the plated layer, parts must be cleaned before plating by chemical degreasing, solvents or electrochemical methods. Therefore the volume of wastewater generated in this process is large but the pollutant concentration is low, mainly alkali, acid and solutions.

2. Composition and properties of plating wastewater

• A defining feature of wastewater from plating production is that it contains many heavy metals and therefore affects the environment greatly, with long-lasting consequences. If it is not treated properly, the metal ions, salts and other compounds released into the environment do not decompose but persist and accumulate in the natural environment.
• This type of wastewater can kill plankton and poison or cause disease in aquatic animals, especially fish (one of the links in the food chain in which humans take part), and in turn cause dangerous diseases in humans. This process is called bioaccumulation: toxins originating from metals, released into the environment, gradually accumulate in aquatic animals over a long time and pass through many food chains, affecting people. A wastewater treatment system that separates the metal ions before discharge to the environment is therefore needed, to meet the regulations in force in Vietnam.
• Plating wastewater varies widely in concentration and its pH ranges from acidic (pH = 2 – 3) to strongly alkaline (pH = 10 – 11). A common feature of plating wastewater is a high content of inorganic salts and heavy metals. Depending on the metal of the plated layer, the pollution source may be Cu, Zn, Cr, Ni, etc., and depending on the type of metal salt used, the wastewater may contain toxic substances such as cyanide, sulphate, ammonium, chromate, etc. Organic matter is scarce in plating wastewater, mostly flocculants, surfactants and the like, so BOD, COD and oil and grease are usually low and are not the main treatment targets. The main targets are inorganic ions, especially heavy metal salts such as Cr, Ni, Cu, Fe, Zn, etc. High pH, high temperature.
• For factories that use industrial salt in production, the wastewater usually has a high content of ammonium and nitrogen nutrients, and may even have high Cl from salt electrolysis. These components are also very hard to treat and costly to treat

Table of common plating wastewater properties:

(*) The pollution values do not include concentrated pollution streams

3. Common plating wastewater treatment technology in Vietnam

• The current plating industrial wastewater treatment system diagram uses chemical precipitation of the metals in the wastewater. This method requires strict compliance with the reaction conditions in order to remove as much heavy metal as possible. The characteristic metals of this stream, chromium, zinc and nickel, are all amphoteric metals: their chemical precipitates can dissolve in excess alkali to form complexes. So if pH is not controlled properly, the metals cannot be removed to the maximum extent and the water quality cannot be improved to meet QCVN 40:2025/BTNMT.
• Chemicals commonly used to treat plating wastewater are FeSO4, FeCl, H2SO4, Na2S2O5, PAC and polymer

Description of a chemical plating wastewater treatment technology

4. NGO’s industrial wastewater treatment solution

• RECOVERY OF PRECIOUS METALS in the wastewater for reuse in production, while reducing the load to be treated at the wastewater treatment plant, making the treatment system more economical
• LOW CHEMICAL USE to remove heavy metals and other pollutants from the water, which reduces chemical costs, makes operation less laborious and less hazardous, reduces the sludge generated and limits secondary pollution. Because no chemicals are used and the process does not depend on pH conditions to coagulate each metal component, NGO’s solution can treat all wastewater streams generated in the plating process WITHOUT STREAM SEPARATION for projects with limited installation space, thereby reducing the number of storage tanks and the complexity of the treatment process, and simplifying the wastewater collection system from the production sections
• WATER REUSE & RECIRCULATION back into production

4.1 Description of the treatment system

4.2 Laboratory test results of the treated water samples.