Anodizing Wastewater Treatment Technology

Anodizing is an electrochemical process that involves treating the surface of a metal, typically aluminum, to form a durable, corrosion-resistant, and aesthetically pleasing finish. This process is widely used in the aerospace, automotive, and defense industries, as well as in many other applications that require high performance and long-lasting durability.

Unless otherwise specified, anodizing typically refers to sulfuric acid anodizing. The oxidation process that occurs when substances other than metals are used as the anode is also referred to as anodizing.

ⅠSources of Anodizing Wastewater

The wastewater from anodizing primarily originates from the degreasing and cleaning processes of aluminum products, the alkaline etching and cleaning processes, the pickling and cleaning processes, the chemical polishing and cleaning processes, the anodizing and cleaning processes, the dyeing and cleaning processes, and the sealing and cleaning processes.

1. Degreasing/Deoiling

• Organic solvent deoiling
• Ultrasonic deoiling
• Electrochemical deoiling
• Alkaline deoiling
• Water-based degreasing agent

（1）Anionic surfactant: releases negatively charged active groups in water.

（2）Cationic surfactants: ammonium salts, quaternary ammonium salts; they have poor detergency but strong bactericidal properties and are effective antistatic agents.

（3）Amphoteric surfactants.

（4）Nonionic surfactants: They do not ionize, have good stability, do not adsorb on metal surfaces, are easy to rinse, leave minimal residue, and have excellent rinsability. They are the most ideal cleaning agents for metal parts.

Ⅱ Anodizing Wastewater Treatment Process

• Segregated collection and diversion

1.Nickel-containing wastewater treatment

After collecting the cold sealing solution, it enters the evaporation system. The cold sealing rinse water is first treated using SRO special separation membranes to produce water that is then directed into the recycling tank. The concentrated water is then concentrated using WEM technology before entering the evaporation system. This process achieves zero discharge of nickel-containing wastewater.

2.High COD wastewater treatment

Due to the high CODcr_{cr}cr​ in the oil, wax removal, and degreasing tank water, pre-treatment is carried out through coagulation and sedimentation. The treated effluent then enters the A/O process to remove COD, and subsequently, it undergoes secondary coagulation and sedimentation before entering the comprehensive wastewater treatment system.

3.Phosphorus-containing wastewater treatment

Phosphorus-containing wastewater is collected separately. The tank liquid wastewater is pumped drip-by-drip into a collection pit, and the electrolytic polishing rinse water undergoes secondary coagulation and sedimentation to remove phosphorus before entering the comprehensive wastewater treatment system.

4.Comprehensive wastewater treatment

After pre-treatment of the comprehensive wastewater using coagulation and sedimentation, it enters the A/O biochemical system for further biochemical treatment. The treated water then goes into the comprehensive recycling system, achieving water resource reuse and energy conservation. The purified water is used as makeup water, while the concentrated water is processed by concentration treatment equipment to meet discharge standards.

• Process flow diagram:

Ⅲ Specialized Processes For Anodizing Wastewater

1.Treatment processes for the first and second rinsing waters after chemical polishing

During the chemical reaction process with phosphoric acid/sulfuric acid, only 10%-15% reacts to form aluminum phosphate and aluminum sulfate, while the remaining 85%-90% adheres to the surface of the workpieces and is carried into the rinse tank.

According to empirical data, 1 ton of phosphoric acid will generate 4-7 tons of sludge through chemical precipitation (depending on the discharge standards).

2.Treatment processes for chemical polishing/anodizing wastewater

Anodizing rinse acid has an acid concentration of around 17-25% and an aluminum content of 10-15 g/L. Conventional treatment processes typically involve neutralization and sedimentation, followed by transferring the sludge as hazardous waste. This approach wastes both aluminum and acid resources and is costly.

• Aluminum removal resin treatment offers high precision, with aluminum content reduced to below 0.02 ppm.
• High adsorption capacity, with a maximum actual exchange capacity of up to 20 g/L.
• High selectivity, capable of efficiently adsorbing aluminum in 15-20% sulfuric acid.
• Highly adaptable, capable of removing aluminum in high-concentration acidic environments (15-20% acid).

• 65-80% phosphoric acid

• 0-10% sulfuric acid

• 2-4% nitric acid（Many processes now do not include）

• 35-45 g/L aluminum

• smoke suppressant

By separating dissolved aluminum from the acid, the acid can be recycled and reused in the process. The advantages are as follows:

• Reduces the need for purchasing acid；
• Reduces the cost of neutralizing spent acid;
• Reduces waste sludge— in some cases, waste aluminum can be converted into commercially valuable by-products.
• Reduces the concentration of waste salts (eg:lower nitrate and phosphate levels).
• Treatment processes for dyeing wastewater