Study on Removal of Organic Pollutants in Electroplating Wastewater
The problem of organic pollution in electroplating wastewater has attracted the attention of the electroplating and environmental protection community in recent years, and its content is usually expressed as CODc. The electroplating industry waste water treatment and monitoring both focus on the removal of heavy metal ions, even if the electroplating wastewater treatment systems currently in operation and construction do not have treatment processes or treatment units for their organic pollutants. After the sewerage standards become stricter, especially after the requirements for full-factor compliance have been put forward, this problem needs to be taken seriously. At present, there are no relevant reference materials and engineering examples on design specifications, textbooks and professional publications on electroplating wastewater treatment. In view of the above situation, the author paid full attention to the treatment of organic pollutants in the design of the process selection and treatment process for the third phase of the Gaoping electroplating wastewater treatment plant in Zhongshan Town, Zhongshan City. Several experiments were conducted with reference to organic pollutant treatment processes in other related industries, and the characteristics of organic pollutants in electroplating wastewater and related treatment processes were initially grasped.
2 Sources of Organic Pollutants in Electroplating Wastewater
There are three main sources of organic pollutants in the electroplating wastewater: electroplating pretreatment process, electroplating process, electroplating post-treatment process.
Table 1 shows the proportion of organic pollutants in electroplating wastewater. It can be seen from the table that the organic contaminants in the electroplating wastewater mainly come from the pre-plating treatment part, and the electroplating process itself occupies a smaller proportion.
2.1 Pretreatment of organic pollutants in wastewater before plating
The CODc in the pre-plating process has a great relationship with the product type and process route of the electroplating company. Workpieces must be surface-refined (eg, polished, rolled, embossed, etc.) prior to plating, and then dewaxed, deoiled, and pickled and activated to provide a clean surface for the subsequent electroplating process. At present, the vast majority of electroplating plants use emulsifying wax removal and degreasing processes, and contain a large amount of non-ionic surfactants, anionic surfactants, and other auxiliary agents (such as corrosion inhibitors) in their wastewater. These substances are themselves high-molecular organic pollutants. In the production process, wax and oil on the surface of the workpiece are emulsified into the working mother liquor, and the oil in the working mother liquor will gradually accumulate to the limit value along with the production process. At this time, the working mother liquor must be completely discharged and then reconstituted. Since the pollutants in the working mother liquor before plating are mainly surfactants, auxiliaries, mineral oil, and waxes, this part of the effluent does not have any recovery value, so all electroplating companies directly discharge them into the sewage treatment system. When the mother liquor CODcr up to 20 000 ~ 50 000 mg / L. Under normal circumstances, the working period of the plating mother liquor before electroplating is 7 to 15 days, so this part of the wastewater is contaminated.
The CODc value of the water treatment system has a strong impact.
Under normal circumstances, the CODc emitted from the pre-plating treatment process is between 100 and 200 mg/L. This part of the wastewater contains a relatively large amount of wastewater, accounting for 20% to 25% of the wastewater discharge, and its load condition is relatively stable.
2.2 Generation of Organic Pollutants in Electroplating Process Wastewater
The organic contaminants in the electroplating process mainly come from various brighteners added in the electroplating bath. These brighteners are multi-component mixed high-molecular organic compounds. The addition amount of a single species is 80 to 150 mt/(kA·h). Generally, two to three varieties of brighteners are added to the plating bath. The workpiece will consume these brighteners during the electroplating process. Some of the brighteners will be decomposed into the plating layer, some will remain in the plating solution, and the other part will be taken out into the cleaning water after being carried out by the workpiece. Therefore, the CODcr in the cleaning water during the electroplating process is between 40 and 60 mg/L. Some types of plating do not contain organic brighteners, such as chrome plating with dilute additives and alkali copper plating with pre-plating parts without brighteners. Electroplating process cleaning water accounts for 60% to 70% of the total discharge, and the CODc of this wastewater is relatively stable.
2.3 Generation of organic pollutants in wastewater after electroplating process
Post-plating treatment refers to a process performed after the workpiece is electroplated, such as dehydration of the surfactant (also known as a dehydrating agent), or a chemical anti-corrosion treatment to increase corrosion resistance, such as copper plating or copper plating of a zinc alloy. Immerse the benzotriazole solution. In addition, during the production process, the defective and defective workpieces need to be removed by plating (that is, deplating). If anti-stained salt is used for deplating (also called nickel-removal powder), the waste water contains a large amount of sodium m-nitrobenzenesulfonate. The effluent was dark brown, resulting in a yellowish coloration of the treated drainage (some would mistakenly believe hexavalent chromium exceeded) and the color was difficult to remove. The CODcr in the wastewater from the post-plating process is relatively large, ranging from 2 000 to 3 000 mg/L for the mother liquor, and 50 to 150 mg/L for the COD 0 in the cleaning water (depending on the process used). This amount of wastewater is not very large, accounting for about 5% of the total discharge.
3 Removal of Organic Pollutants in Electroplating Wastewater
In view of the composition and physicochemical and biological characteristics of organic pollutants in pre-plating wastewater, the author conducted three kinds of process experiments, namely, biochemical method, microwave chemical method, and physicochemical method.
The sewage is adjusted to pH 8 to 9 in the physico-chemical treatment tank, and the heavy metal ions and a small amount of organic substances are removed in the sedimentation tank after adding the coagulant, and then the microorganisms in the pool are degraded in the anaerobic tank, and the In the oxygen pool, the organic matter is further degraded by the meta-oxygen-type microorganisms, and the organic matter is metabolized and decomposed by microorganisms in the later-stage treatment process, and finally settled out in the form of flocs with the death of microorganisms.
However, taking into account that the pretreatment wastewater may contain heavy metal ions and anionic surfactants, which have a toxic effect on bacteria, a set of physicochemical devices should be added before biochemical treatment to remove heavy metal ions from the wastewater and stabilize the pH of the wastewater. Considering that the organic pollutants in the wastewater before plating are mostly surfactants and mineral oil, the biodegradability is very poor, and the impact resistance of biochemical treatment systems and the reliability during operation are considered comprehensively, the author believes that this treatment should be adopted. Process is not the best choice, there is a higher risk.
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