How is coagulation and sedimentation used to treat wastewater?

Coagulation and sedimentation can effectively remove suspended solids (SS), colloidal particles, and some dyes and auxiliaries from printing and dyeing wastewater. The key to this process is the use of chemicals to agglomerate fine particles into larger flocs, which are then separated by gravity settling. The following are the specific steps and technical highlights:

Coagulation and Sedimentation Process

Wastewater Pretreatment

Grate Filtration: Removes larger impurities such as cloth scraps and fibers.

pH Adjustment: Printing and dyeing wastewater is typically alkaline (pH 9-12). Acids (such as sulfuric acid) are added to neutralize or slightly acidify (pH 6-7.5) to enhance the effectiveness of coagulants.

Adding Coagulants

Common Coagulants:

Inorganic Salts: PAC (Polyaluminum Chloride), Aluminum Sulfate (Al₂(SO₄)₃), Ferrous Sulfate (FeSO₄).

Organic Polymers: PAM (Polyacrylamide, available in anionic, cationic, and nonionic forms). Dosage Order: First add the inorganic coagulant (e.g., PAC), then the PAM coagulant.

Mixing and Reaction

Rapid Mixing (1-2 minutes): Agitation (mechanical or in-line) is used to ensure thorough contact between the chemicals and the wastewater.

Slow Flocculation (15-30 minutes): Slow agitation (20-40 rpm) promotes the growth of alumite flocs.

Sedimentation and Separation

Sedimentation Tank Type: Cross-flow, inclined plate, or radial flow.

Retention Time: Typically 1.5-4 hours. After sludge sedimentation, the supernatant is transferred to the next treatment unit.

Sludge Treatment

Sludge Thickening: Static settling or mechanical thickening.

Dewatering: Dewatering is performed using a plate and frame filter press or centrifuge. The sludge cake is transported for disposal (possibly hazardous waste).