There are many methods for treating wastewater. Different methods are required depending on the source. Specifically:
1. Degreasing Wastewater: Treatment methods for degreasing wastewater include acid extraction, centrifugation, or solvent extraction. Acid extraction is widely used. H₂SO₄ is added to adjust the pH to 3-4 to break the emulsion. Steam is introduced, salt is added, and the mixture is stirred. It is then allowed to stand at 45-60°C for 2-4 hours, allowing the grease to gradually rise and form an grease layer. Grease recovery can reach 96%, and CODcr removal is over 92%.
2. Lime Lamination and Hair Removal Wastewater: Treatment methods for lime lapping and hair removal wastewater include acidification, chemical precipitation, and oxidation. Acidification is commonly used in production. Under negative pressure, H₂SO₄ is added to adjust the pH to 4-4.5, generating H₂S gas, which is absorbed by NaOH solution to produce sodium sulfide for reuse. Soluble proteins precipitated in the wastewater are filtered, washed, and dried to become the final product. Sulfide removal rate can reach over 90%, with CODcr and SS reduced by 85% and 95%, respectively. It is low-cost, simple to operate, easy to control, and shortens the production cycle.
Sulfur-containing wastewater can be treated using methods such as iron salt sedimentation, chemical coagulation, and catalytic oxidation, with catalytic oxidation being the most common pretreatment method.
3. Chrome Tanning Wastewater
Treatment methods for chrome tanning wastewater include alkaline precipitation and direct recycling. Most tanneries use alkaline precipitation, adding lime, sodium oxide, and magnesium oxide to the waste chromium solution. After reaction and dehydration, chromium-containing sludge is obtained, which can be dissolved with sulfuric acid and reused in the tanning process.
Chromium-containing wastewater should be collected separately, precipitated with alkali, and then separated into sludge and water. The supernatant, meeting the standards in GB30486-2013, is discharged into a comprehensive wastewater equalization tank. The sludge from the sedimentation tank is discharged into a chromium-containing sludge thickening tank, dried by a filter press, and then disposed of by a qualified hazardous waste treatment unit.
4. Integrated Wastewater
Pretreatment System: This mainly includes treatment facilities such as screens, equalization tanks, sedimentation tanks, and dissolved air flotation tanks. Tannery wastewater has high concentrations of organic matter and suspended solids. The pretreatment system is used to regulate water volume and quality: remove suspended solids (SS) and reduce the pollution load, creating favorable conditions for subsequent biological treatment.
Tannery wastewater has good biodegradability, therefore biological treatment is the preferred process. However, before this, chromium-containing wastewater, degreasing wastewater, and sulfur-containing wastewater must be collected separately and pretreated.
