Common methods for urban wastewater treatment include biological and physicochemical methods. Biological methods are more widely used 

and have higher efficiency. Flocculants can be used in various stages of biological treatment, such as in primary and secondary settling tanks and 

thickeners. However, the most important application of flocculants is to aid and promote sludge dewatering. During sludge dewatering, flocculants

cause the sludge to form large and dense flocs, which can be separated from the clarified liquid using centrifuges, belt filter presses, or plate and 

frame filter presses.

Selecting the right flocculant is one of the keys to success. When choosing a flocculant type, the following factors should be considered:

1. Understand the source, properties, composition (e.g., organic/inorganic ratio), and solids content of the sludge from the user. Generally, cationic 

flocculants are used to treat organic sludge, while anionic flocculants are used to treat inorganic sludge. Cationic flocculants are not suitable for 

highly alkaline conditions, while anionic flocculants are not suitable for highly acidic conditions. Sludge with high solids content usually requires a 

larger dosage of flocculant.

2. Floc Size: Flocs that are too small will affect the drainage rate, while flocs that are too large will bind too much water, reducing the dryness of the 

sludge cake. Floc size can be adjusted by selecting the molecular weight of the flocculant.

3. Ionicity of the Flocculant: For the sludge to be dewatered, flocculants with different ionicities are screened in the laboratory to select the most 

suitable one. This achieves the best flocculation effect while minimizing the dosage.

4. Floc Strength: Flocs should remain stable and not break under shear stress. Increasing the molecular weight of the flocculant or selecting a 

suitable molecular structure helps improve floc stability. 5. Mixing of Flocculant and Sludge: The flocculant must react fully with the sludge at a 

specific point in the dewatering equipment to achieve flocculation. Therefore, the viscosity of the flocculant solution must be appropriate to 

ensure uniform mixing with the sludge under existing equipment conditions. Uniform mixing is another key factor for success. The viscosity of the 

flocculant solution is related to its molecular weight and concentration.

6. Dissolving the Flocculant: Good dissolution is essential for effective flocculation. Sometimes, it is necessary to accelerate the dissolution rate. In 

such cases, increasing the flocculant solution concentration (higher concentration leads to faster dissolution), using emulsion-type flocculants, or 

appropriately increasing the water temperature can be considered.