flocculant, coagulant, PAC, PAM, polyaluminum chloride, polyacrylamide, water treatment, pH value, water temperature, impurities, dosage, hydraulic conditions, sedimentation, wastewater treatment, polymer flocculant

The performance of flocculants is influenced by various factors, including water pH, temperature, impurity composition, flocculant type, dosage, dosing sequence, and hydraulic conditions.
The pH value of water plays a crucial role, especially for inorganic flocculants. It affects hydrolysis speed and the type of hydrolyzed products formed. Taking aluminum salts as an example, when pH < 4, Al³⁺ exists mainly in ionic form and does not effectively hydrolyze into Al(OH)₃, resulting in poor coagulation. At pH 6.5–7.5, neutral colloids of Al(OH)₃ form, providing optimal performance. When pH > 8, Al³⁺ converts into AlO₂⁻, and coagulation becomes less effective. If alkalinity is insufficient, lime can be added, while overly high pH should be adjusted using acid. In comparison, polymer flocculants are less affected by pH.

Water temperature also influences hydrolysis and floc formation. Since most coagulation reactions are endothermic, low temperatures slow down hydrolysis and reduce particle collisions due to higher water viscosity. Conversely, excessively high temperatures can degrade polymer flocculants, weakening performance.

The composition and concentration of impurities also matter. Uneven particle sizes promote flocculation, while overly fine or uniform particles reduce efficiency. Low particle concentrations can be improved by recycling sludge or adding coagulant aids. Organic matter reduces coagulation efficiency, requiring higher dosages or oxidizing agents. Calcium, magnesium, and phosphate ions usually enhance coagulation, while certain anions and surfactants inhibit it.

Choosing the right flocculant type depends on the water’s characteristics. Inorganic flocculants are suitable for colloidal impurities, while polymer flocculants or coagulant aids can improve fine particle aggregation. Combining inorganic and polymer flocculants often yields better results and broader applicability.

Flocculant dosage should always be determined experimentally. Excessive dosage can restabilize colloids. Typical dosages range from 10–100 mg/L for aluminum or iron salts, half to one-third of that for polymerized salts, and 1–5 mg/L for polymer flocculants.

When using multiple flocculants, dosing order is also important. Generally, inorganic flocculants should be added first, followed by polymer flocculants. For larger suspended solids (>50 μm), the sequence can be reversed to improve bridging and destabilization.

Finally, proper hydraulic conditions are essential. Rapid and uniform mixing is needed initially, followed by gentle stirring to promote floc growth and prevent breakage.
Combining PAC and PAM significantly enhances dye wastewater decolorization. PAC compresses the double layer to form fine flocs, while PAM’s long-chain structure bridges and strengthens them. The amide groups in PAM also form ionic bonds with dye molecules, facilitating floc sedimentation and improving treatment efficiency